T6 and T10 are my favourite turns because of the mechanics, the eat/spit and wild charge in particular. I also like when we have to protect something like in LotA or Tam Tara HM. I actually wish sometimes we had a NPC to escort through a dungeon instead of them waiting at the entrance for us to return (Byraflox kinda had this but it was impossible for Brayflox to die, so it doesn't really count). They could be part of the dungeon's boss strategy, where we have to protect them but they could also help us with their own special technique (borrowing from another SE games, the bit in KH2 where you have to escort Minnie to the throne room is one of my favourites. You have to protect her but she can also use Holy).

WoD has some fun mechanics too. I remember suggesting something based on colours a long time ago so I was glad for the hot/cold floor during Angra Mainyu, getting inside Cerberus to kill its stomach is my favourite bit in the game,.

I also like weird debuffs that you get in Gilgamesh, Ultros and Wanderer's Place HM, and I have a soft spot for Garuda Ex's spiny plume. That was the first difficult fight I had unlocked and it was a mechanics so out there compared to what I had seen so far, I was very surprised!

Also, I thought it might be fun to have the party split up individually or in pairs at some point to accomplish a task.
I have though about this too a couple of times. Or, if you fall from the arena (think Demon Wall), instead of dying, you'd end up in another place and have to get back up and join the group.

Jumping puzzles/platform mechanics are always fun too. And I mean true jump puzzles, not the platforms we have now that jump for us.