Don't have too many coherent thoughts on crossovers right now, but could contribute on AUs a bit.
Got a few silly joke ones, one that's more serious and another one that's jokey but also a lot more fleshed out.
Pseudofell! – All the characters look like (and might superficially act like) their Underfell selves, but actually have their canon personalities. Probably just popped into my head because of this pic:
Gangstatale/Underhood! – Where everyone acts like an OTT ghetto gangster stereotype, presumably because some dumped too much '90s rap music into Mt. Ebbot.
"Yo dawg, it's yo top homie Flowz Da Flowa! Does a brotha want some 'Friendliness Pellets'?"
Undertaletale! – All the characters play the game and participate in the fandom! Ones wonders how Chara and Asriel would feel about being depicted as an omnicidal demon and a sociopathic flower, respectively…
And a random unnamed idea where Frisk subdues everyone by showing them Undertale porn instead of Fighting or Sparing. I suspect it would end with the monsters deciding they want nothing to do with the surface world anymore…
This was inspired by the wording of the rule about needing a Monster and Human soul to leave the barrier. It didn't seemed to lay out explicitly that one had to absorb the other… Of course that IS the case, otherwise Asgore wouldn't off himself to let you out.
…But what if it wasn't? What if one you could have a Monster and Human leave together, or something like that?
Of course, there would need to be further modifications, otherwise Chara could just escort everyone out one-by-one and then we're just left with the story of chocolate-obsessed brat who's upset about how much his feet hurt, and I don't see a lot of demand for that.
But maybe, if they didn't know this was possible or were mistaken about what it took, there could be something here… Or maybe it's an awful dead-end of an idea, I dunno.
TRIPLETALE!
Basically, what if Vanilla Undertale, Underfell and Underswap happened at once? Like, all three versions of the characters in each "slot": All three Frisks fell down and meet all three Floweys before getting taken in by all three Toriels, etc.
I do have a bunch of thoughts for this I can go into if anyone gives a crap, but otherwise I'll stop here.
I do get the feeling that everything I just said has been done or is just stupid and horrible, and I wasted my time with this post but screw it, this thread's been kind of slow lately. At least tearing apart my dumb ideas would give us something to talk about, I suppose.