Blizzard recently stressed that using any mods in Diablo IV is forbidden. All mods, no matter how benign their effects, are treated the same as cheating, and...
Blizzard says that modding Diablo IV can result in a permanent account ban::undefined
Project Diablo 2 is still going strong and has a lot of nice QoL features and skill improvements over D2 and D2R. You should check it out if you're a D2 fan like me.
Grim Dawn was my Diablo 2 fix. It runs decently, has a dark atmosphere (though if you want story and lore you'll have to read a lot in-game) and cool items.
Also a massive world, especially with the DLCs. Definitely worth playing in my opinion :)
You get nearly free unlimited respecs, the only thing you can't undo is your 2 classes you choose.
You can still try the earlier Diablo games. Diablo 1 with Belzebub mod is pretty fun (or get DevilutionX if you want a more authentic experience but with some modern QoL stuff), Diablo 2 with Project Diablo 2 mod is even more fun (probably the best Diablo variant IMO), and there's also Blizzard's official Diablo 2 Resurrected, if you can't tolerate old school graphics.
Yeah, if you've never stepped foot in the Diablo world don't bother. I had a good time with the D4 campaign, I actually think that part shines but it's maybe 30 hours of gameplay at best, even if you take a few detours along the way. Pretty steep price to game ratio.