In general, I'd like to see more "chaotic" mechanics or things with less strict patterns. I think my favorite singular mechanic in all of Endwalker was during the fight with Shishioji on Mount Rokkon where dozens of very thin line AOEs fire across the arena in all different directions, creating a large, semi-random pattern that can be easy to get clipped by. I want more mechanics like that--stuff where it's possible to avoid, but expected that you will take a few hits and to not punish the player for taking some. Another similar example from Shadowbringers would be E5 Normal, where the battlefield gets sprinkled with a bunch of small, briefly telegraphed circular AOEs where lightning strikes, and you simply dance around them.

I want fights to feel more like I'm battling against a threat, not performing a dance recital with my opponent. It stops feeling like I'm fighting things. Or where it feels like there's an actual battle going on because there are multiple smaller things happening all over. Thornmarch is another example. I know people never liked it because of how you had to lower all the moogles at once, but I really liked the actual mechanics of that fight.