Hope I'm right here with this, sorry if the question is too specific for this community.
Our long-term D&D campaign has reached PC level 17. Now's the time for implementing mechanics that fasten up combat (we all don't like how long it takes), as well as addressing spells that the GM (me) fears to be game-breaking (mostly 9th level spells, full list below).
Sooo... I'd be thankful for any helpful suggestions. My current ideas for the combat aspect include shock damage, Instinct and Morale checks from Worlds Without Number, but those probably don't suffice yet. And I'm having trouble conveying to the spellcasters that toning down some spells is necessary in order to maintain a functional game that doesn't tear apart hours of preparation every session. Feel free to point out any potential counter-arguments to my view on that as well.
I'm really determined to keep this game running. Thank you all so much in advance!
List of spells that require revision or more exact definition (in my opinion):
Wish, Meteor Swarm, Invulnerability, Gate, True Polymorph, Leomund's Tiny Hut, Rope Trick
The biggest challenge during Tier 4 is still resource attrition. Let them use their big spells, but don't let them rest. The best challenge you can give them at this point is to make a multi-session-spanning dungeon-like structure.
An example from my previous campaign: heroes needed to get to the lowest level of Hell, but they needed to transit through every one of them in process. Enemies were everywhere, and places for rest were virtually nonexisting. I think they had like 1 long rest in four months of play during T4, and it actually was hard for them.
Did one of your spellcasters know the Gate spell? If so, what stopped the PCs from pulling the BBEG out of their lair and beating them up from the comfort of their homes? I'm having trouble coming up with challenges that don't actively oppose the abilities of high-level PCs. Just saying 'no' and refusing to let a spell take normal effect would very likely seem unfair, no matter how logical the in-game explanation might seem.
Did one of your spellcasters know the Gate spell? If so, what stopped the PCs from pulling the BBEG out of their lair and beating them up from the comfort of their homes?
Anti-magic field?
The BBEG has taken great efforts to hide their true name?
Private Sanctum spell?
The fact that Gate is an inter-planar spell, and not an intra-planar spell?
As the GM, one of your jobs is to present challenges for the party to overcome. That means actively countering their abilities list just as often as it means giving them a bone and letting them shine. The big moments of flexing mean a hell of a lot less when all they do is flex.
Our Sorcerer knew Wish, but the player knew better than to try something like wishing to get to the lowest level of Hell, because on the meta level they wanted to play through this adventure, not to cheese.