Edit: as a little clarification, this is just a fun topic I like to think about and hear other fun ideas, nothing more.
SInce we are all throwing around inspirations about this and ideas about that, I thought. Let's do something less graphical, because I'm genuially no fun and wrant to write huge walls of texts.
Let's talk about something that will keep you engaged differently then being awed at how good/terrifying it looks.
Boss mechanics.
(sorry for the horrendous quality but it didn't want to check 50 videos to find one without talk over commentary)
https://www.youtube.com/watch?v=b496Mc9A4U0&t=53s
Maybe not the greatest boss mechanic of all time, but it was funny and different, so that's all this thread will be about.
Throw me your most stupidly fun boss mechanics you can think off.
One I always had was for a Boss that's not so much about killing, but keeping it in check, maybe even just an "unbeatable" add that has to be controlled and kept at bay, not killed.
Illustration below

Just imagine a huge room with lots of pillars, a Boss guarding an entrance, chains that keeps him in the room, but can stretch a long long time.
You aren't gona fight him really, but he's gonna enrage on you, chase you and you somehow have to navigate him around the room around those pillars until the chains can't stretch any further. He also has to be far enough away from the entrace now so you can pass. (insert some magical blabla like his proximity gives the entrance a magic barrier)
I would add to that that you can only cross chains on certain conditions, but in general they are gona decrease the "effective room size" you can use again and again. (old spider vs snake game anyone?)
So yeah, a boss that's not about killing, but figuring out what the clear condition is, and how to achiev it.