What RPG have you been dying to run?

So many. And the answer to all "why nots?". Time. It's time. So off the top of my head
Eat the Reich - "The year is 1943. You are a team of crack vampire commandos with one mission: drink all of Hitler's blood"
Conan 2d20
Legend of the Five rings (5e)
Stoneburner - Deep Rock Galactic the TTRPG
Vaesen - Call of Cthulhu but rooted in nordic mythology
Heart the City Beneath - an award-winning complete tabletop roleplaying game about delving into a nightmare undercity that will give you everything you’ve ever dreamed of – or kill you in the process.
- I haven't played Stoneburner yet, but I ran a 10 session campaign of Tales of the Burned Stones (the free fantasy version released as a reward from the Stoneburner Kickstarter), and it was super fun! If you get a chance I highly recommend bumping Stoneburner up in your queue.

Lancer, I need to finish my Curse of Strahd campaign first...
- I'veran a beam saber test game, and plan a campaign soon. If you're not hyped by D&D but with mecha, it May be a solid alternative
- I need to finish my Curse of Strahd campaign first
  That's my reason too.
  
  Is the game you'd like to run also Lancer? That would be a wild coincidence.
  Edit: Oops, nevermind, I just saw your other comment!

Paranoia. I’m afraid I’ve hyped it up too much internally and there’s no way any real play will meet my expectations.
- High expectations are considered treason, please consider a more reasonable expectation before attempting Paranoia

I'd love to play in a Vampire: The Masquerade campaign, or to GM an Avatar Legends campaign. But I've only got one gaming group, we've got one ongoing campaign, and even getting them to play Pathfinder rather than D&D was an unusual and new experience for them.
- We just wrapped up an avatar game. It has some oddities but overall really fun!
  
  Oh nice! Good to hear. What were your thoughts on the system more specifically, if you don't mind my asking?
- Are you me? This is almost my exact situation. Only difference is that I convinced them to PF2e from The Dark Eye 5 (DSA5).

Any gumshoe game, probably something shorter than Nights Black Agents: The Dracula Dossier. If I set it in my own setting, I'd like to use Bubblegumshoe to do my own telling of "Tomorrow When the War Began" basically what happens if on the summer camping trip after your last school year, your country is invaded. I can't quite tell how good Gumshoe is for homebrew settings however.
My other want is to run a worldbuilding game such as the quiet year, for the queen or microscope, hacked to set up a concise and thematic noir mystery inspired by fiction like Disco Elysium, The City and the City or The Nice Guys, with a rich and vibrant world that the players are invested in as they built it. I'm tempted to hack the bladerunner RPG by Freeleague for the actual police procedural afterwards.
- a worldbuilding game such as the quiet year, for the queen or microscope, hacked to set up a concise and thematic noir mystery inspired by fiction like Disco Elysium, The City and the City or The Nice Guys
  That sounds amazing. I love the thought of a Nice Guys-inspired campaign setting.
  
  Seconded! OP, in a similar vein as the world building games you already mentioned, you might find Intrepid interesting. I could see it being used to do world building + history of some of the major players and nations therein.
  I could also imagine the relationship map it uses being hacked a bit to allow for some Disco Elysium style personality skills / thought cabinet shenanigans if you were interested in leaning into that in particular.
  
  Thank you, I've realised that my approach seems a little different from other here, where I try to pick an RPG for an idea that's forming in my head, based on the genre and tone, settling on an RPG that's 80% there but people love the ruleset, then I chop and change it to get close enough to 100%.
  This is probably detrimental in a few ways too, as some games like Lancer are unchangable until I'm familiar enough to peel apart the interwoven mechanics and lore, and I'm not going to touch it because I almost never run official settings and adventures, particularly in longform games.
  I will shout out both Alice is Missing and For the Queen, which both get worse when they get altered, because their strength comes from their simplicity and then probably ridiculous amount of playtesting.
- Ive run a few gumshoe games. They are great, but did require a lot of planning on my part.

Three-way tie. Unfortunately there hasn't been movement on any due to personal stuff, but hopefully soon:
Pathfinder 2e for a modern D&D-type experience. (Not to yuck anyone's yum, but I have plenty of gripes with 5e.)
Dolmenwood. Currently awaiting delivery of the Kickstarter. For those old-school D&D vibes.
Burning Wheel. My favorite game I've never played, even after owning the books for nearly two decades. :P But for real this time!

Mage the awakening 2e. I had a game of it for about 20 sessions, but it fell apart. One player never really learned the rules and was easily frustrated, which is a bad combination. She also straight up said she wasn't going to read a 5 sentence paragraph clue. I definitely had some fumbles, but chief among them I think was overestimating how much they were willing to think about clues and text. They wouldn't even taken notes between sessions, and then would forget.
I also want to do Fate, but it's been hard to find a group. I got my old DND group to try it, but I didn't really like playing with them. I have a new group, but someone just dropped so now we're down to 2 players again. It's a nice game though.

Electric Bastionland has been the RPG I most want to run ever since I bought it years ago.
After watching a surreal Acid Western (which started out deceptively straightforward and veered endearingly off the rails by the final act), I'm itching to run In The Light of the Setting Sun: Bonanza Edition. But I'm at the end of my current FIST: Ultra Edition campaign and don't want to rush anything. :D
- What was the film? Enquiring minds want to know.
  
  It's called Walker (1987). Directed by Alex Cox (Repo Man) and starring Ed Harris. Not currently streaming anywhere I know of, but if you can find it on DVD or something I highly recommend it. The less you know going in the better. :)

I've always loved Shadowrun's lore. I'd love to play it, but I don't think I'd enjoy the crunch.
I borrowed Blades in the Dark flashback mechanic for a 5e heist. It worked out well, so I'd like to try the full game.
Ten Candles also sounds pretty cool. But I play exclusively online, so that won't work for now.
- Ten Candles is an absolutely incredible experience. Now that I've done it once I dunno if I would want to do it again, but I'm really glad to have done it. Like you said though, it really needs to be IRL.

Anything sci-fi, using the Savage Worlds system.
I can't find players. Locally, I can't find anyone playing anything other than 5e. Online, I've been able to find players for various non-5e systems, but haven't managed to organise a sci-fi SWADE campaign (yet).

wanderhome! i don't have a group of friends who are ~~cringe~~ creative enough to do a fully nonviolent game

Some day I'll probably run Fate. I'm currently playing Fate as a player, but some day when that campaign's over I'll probably start a Fate game as a GM.
As to the "why haven't I run it yet", mostly because it wasn't really on my radar until recently. Once it was on my radar, I happened to stumble across someone else advertising to run a Fate game, so I focused my energies on getting into that game as a player.

I've really wanted to learn Delta Green so I can run a SCP story I love (What Happened to Site 13?) and at the end the site teleports out of the world.
Then I'd run a fantasy campaign that is cleaning up the mess they left in the Delta Green one.

Soooo many. TTRPGs are a hobby I adore despite not playing much of lol.
Books I own that I haven't played yet include most of the Chronicles of Darkness series (I'd love to try the new Vampire: the Requiem 2e, Werewolf: the Forsaken 2e, Promethean 2e, Changeling 2e, and Mage: the Awakening 2e especially), Dungeon World, Blades in the Dark, the Avatar rpg, and Shadowrun (don't remember).
I've also been interested in a more crunchy D&D than my normal game 5e, so I've been looking into Pathfinder 2E and D&D 4E. I've been actively looking into running and playing in 4e lately and so far, reading into it, I don't think I have it a fair shake the first time around. I also wouldn't turn away Level Up 5e if someone offered to run it for me.
Other games I'd like to play: Vaesen, Mothership, Paranoia, and now Daggerheart.

I have lots, but the top of the list is probably Cairn - doubly so now that the Cairn 2e Kickstarter is out. It seems like the perfect mix of light mechanics and shenanigans.
Like many others, I haven't played because my group is in the middle of a multi-year D&D campaign. That, however, got put on a deadline, as our second child will be along later this year, so who knows what the future holds?

I'd love a go at Crown and Skull by Runehammer. It looks really interesting. I'd like to play it before I run it and, frankly, just don't have the time.
- I was looking through the SRD of 13th age and found it intriguing. Later on I went to the new second edition Kickstarter. It doesn't have a playable preview, but in the one they have is very interesting the sort of changes they are highlighting,

Mouse Guard, definitely. Lack of time and energy, mostly.

Ahhh, loads of them!
D&D adjacent stuff like Pathfinder 2e.
PBTA and kin like Dungeon World, Monster of the Week, Ironsworn. Blades in the Dark.
Delta Green.
I have a whole folder of freeform, GMless stuff like Microscope, Kingdom, Follow, Archipelago, Dawn of Worlds, Dialect, Intrepid, The Quiet Year.
Gonzo one shot stuff like Paranoia, The Sorcerer Supreme, Ten Candles, peace was never an option (think untitled goose game: the rpg).
I recently had a test run or two of Fate Condensed and got completely hooked.
Most of those I'd be content to run or play, but the game I really want to GM myself is a campaign I've been dreaming up for about year now that I call the "sedition sandbox". The scope is focussed to a single city, the capital of a hostile foreign empire. The PCs are an elite team of saboteurs and infiltrators, sent by a nation desperate to turn the tides of a losing war. Their only objective is to bring down the empire from within. Intrigue, plots, rebellions, sabotage, faction politics, assassinations, propaganda, blackmail, anything the players can think of to achieve that goal is fair game. How far will they go to stop the greater evil?
I think it would be a blast (naturally, I guess), and one or two players in my local network have expressed a little interest, but I've yet to rally the kind of commitment I think I'd need to really get off the ground. And the inexorable march of time makes it less likely with every passing day, as my friends either move away or are subjected to all life's little tyrannies of responsibility ¯(ツ)_/¯

Eclipse Phase. It's a d100 body-swapping cyberpunk scifi horror game partly inspired by the Altered Carbon novels. In the default campaign the players are part of a conspiracy to steer humanity away from tampering with X-threats like alien viruses and tech left behind by hostile superintelligent AI.
I still might run it. My group cycles through GMs, each one of us running a campaign that can be years long. Last time it was my turn I ran Genius: the Transgression, next time could be Eclipse Phase.

Paranoia, Don't have anybody that would want to play

Shadowrun.
Even before scheduling conflicts halted our D&D sessions, none of my friends wanted to play Shadowrun even though I am down to DM. We all played once, and while I fucking loved it and crave to play again, apparently nobody else did.
- I have had problems with players saying that they were down for it, and they understand that they need to learn the rules, but then they play a technomancer and not really know what a sprite or the matrix is.
  I liked the game, but I hated the broken promises.
  
  I do think the rules are harder to understand because the books are just awful in the layout and presentation. I had to use exterior resources to understand character creation, magic and the "wireless world" stuff for 4e, and found that the previous version actually was simpler to understand (and I just find the need to physically jack in more fun than Watch_Dogging that shit).

Starfinder. I was looking into adventures with Starfinder 2 was announced, and just shelved it, because of course I'd rather play the new, shiny space game, rather than the one built on a 20 year old chassis.

The MCDM RPG, it's not out yet. (Patreon backers have access to playtest documents, but I'm not that dedicated.) Also it has a name, we just don't know it yet. No, I'm not salty about that.
"Bite Marks", a PBTA game about werewolves. Just hasn't been the right time yet.
Savage Worlds variants include Weird War II, Weird Wars Rome, Last Parsec, Legend of Ghost Mountain--in each case, I only have limited game time, so there's just a big old backlog, you know?
- I picked up all the Last Parsec books last year. Now they sit sadly on my shelf. Wish I could find some interested players.
  
  I'll do you one better, I have the books (except for Irongate), AND the Action Decks, AND the custom Bennies!
- “Bite Marks”, a PBTA game about werewolves
  Gonna be honest, this is the first time I've heard of it, but I hate the name. That's clearly the name for a vampire RPG.
  
  I'm not surprised, I don't recall how much the kickstarter raised, but it wasn't a ton. (Still enough to fund, obviously.) It was originally called "Bite Me!" (which is approximately as vampire-y, though I certainly liked it a lot better) but somebody had a copyright on that name.
  Anyway, here's the DriveThruRPG listing

The wrath of the white witch...
I am almost done with college forever (probably hopefully).

- This community is for "Pen & Paper" or "Tabletop" RPGs.
  
  Sorry for the confusion. TBF, a lot of the games I'm seeing in the comments were made into video games as well, which doesn't help. The name of this community isn't helping, either, lol. Probably should have been called something like tabletop_rpg instead, since these days most people associate RPGs with the video game form factor.