Cure wounds.Is there any spell where this would just be utterly broken to use it on?
Is this actually necessary? Even summoning an army of zombies didn’t seem they out of whack for the 9th level cast, are there long duration spells we have not considered?Add a line saying that when "Binding of Infinite Power" ends, all spells cast with it also end.
Counterspell is an action economy crusher more than a slot issue.I'd say Counterspell jumps out to me. A 20th level caster counter spelling every round would be an issue.
Counterspell is an action economy crusher more than a slot issue.
By level 17, you have a lot of slots for counterspell, and burning a slot to make the enemy burn a slot and an action already has such insane ROI that I'm not sure making it at-will matters much.
Unless you have a long encounter where there is a constant stream of spellcasters to counter.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.