Do you expect any deaths in the final fight? I'm kind of split on that personally. As DM it's your job to make sure your players enjoy themselves and it's hard to enjoy a game without stakes. However, if you've taken a character that far only to die in the final fight, it can ruin the experience for that player. It's especially bad when you reach that point, hit a string of crappy luck, have some bad rolls and have to watch your character die, or worse, a TPK.
My current plan is to only tweak fights on the fly if I feel like I screwed up in balancing them. If I have a completely underpowered or overpowered creature that is entirely on me, I might fudge the numbers a bit to balance the encounter but that's about it. If a character dies, they die. It's the nature of the game.
I hadn't read about Sigil but I'm definitely incorporating it now.
You read my mind about Mechanus. I'm having a fortress/small city that was pulled from Mechanus. It's inhabitants will be malfunctioning which could make for some interesting encounters. The party will meet several NPCs who are trying to set things right. The ritual that created the Island will also have messed with the planes. Unstable portals between planes appearing and disappearing, creatures being corrupted as one plane bleeds into another, etc.
I'll definitely look into the Clockwork Soul Sorcerer. The party will probably need some help along the way as there are going to be some pretty nasty creatures coming in from other planes. For example, they're going to face a Kracken that came in from the water plane at some point.
Next time I play a character I'm going to try to build a character around crits. I'm still looking into it but it's going to be some kind of dex based elf/half-elf champion fighter. At 3rd level you can crit on a 19. Take elven accuracy and use finesse weapons so when I have advantage I can re-roll one dice. That would give me 3 shots at a crit whenever I have advantage. Still working on how to consistently get advantage. I would do a barbarian dip to get reckless attack but it doesn't work with elven accuracy as reckless requires the attack to be strength based and elven accuracy doesn't work on strength based attacks.