tbh Kai and Rin could fit in various base-spots too
They could easily be like Chara and Asriel— though that would bring the question of who’d be in the basic Toriel and Asgore spots, but. It’s a feels-fitting thing. …. also I guess that’d imply a Flowey Rin which… sounds sad.
Kai could be a Toriel since they can be the mom-friend but also— protective, potentially overly so— or Asgore given as well, like they were for a while in the Mint AU before Len usurped them.
The Sans placing I think mainly came from the ‘remembering timelines’ thing? And base idea of a battle with them when they’re actually trying being Really, Really Hard and dying a lot during it would be fitting. And then it just kinda expanded from there.
Mettaton’s a bit of a joke thought, since they can be flamboyant as well, and mainly just goes with looks since, well, thanks to Mettaton’s hair and how people drew it Kai’s hair looks as it does now,
Kai in a Flowey position could be possible too even if not in an Asriel situation, given their corruption. They’re in a Flowey position in the Rook AU, and Rin’s like… a mix of Toriel and Asgore in it too.
Rin could be in a Toriel place, like she is in the Mint AU and kinda-Rook AU, partially since it’s cute to imagine her guiding whoever’s in the Frisk-place through the ruins, along with also wanting to take care of whoever falls there, and also not wanting them to get hurt or killed…
Alphys is a slight possibility too with the geekiness and anxiety– though with everything else, that’s sad to think about…
I could see her in a Papyrus-like role too, if Kai’s in a Sans one. though I guess also if Len’s in that spot too, but either way. Less I’M THE GREATEST and more just generally peppy though, probably…
and I suppose this isn’t even considering the various swaps that are around Undertale, though if there’d be swaps to other roles I’d prefer it be more inverted fate-y as opposed to totally switched personalities or somethin’