Underfell:

- A Guard Batallion Battle (Just a simple idea, there would be Royal Guards in The Ruins who got locked up before they finished their mission and couldn't leave The Ruins just like that.)

- Napstablook as a Spirit Warrior Fight (Weird idea, but what if his cousin encouraged him more and he was more confident and became sort of a guard, but not a royal one? It's an AU after all, you can do anything as long as it's the character. He'd wield two swords in his ghost hands appearing only when he chooses to.)

- Muffet Fight (freaking please. Canon-like or not, please.)

- Undyne Fight (not Undying) + The "Date" Sequence (not hard to do really, just edit the original Undyne house if you can't get too fancy)

- Toriel Fight + possible chase sequence (you can literally make your own version, that's.. AWESOME. why don't you?)

- Papyrus Fight + The Date Sequence (Your version or any at all, just original and good.

This is literally the most based and the best idea ever if you actually got to work and made it.)

- Sans Fight (nor too edgy, nor emotional, just make something completely new.)

[someone already made a game page with stealing my idea for that emotional fell unfortunately,

that music theme was meant to be made for The Ruined Underground and they made a game instead.. using my idea,

thats why I uhh love to work alone,

people are asses sometimes,

I uhh advice you to not get help,

don't. just work alone.

especially if you're not a straight, based, "normal" person.]

Underswap:

- Alphys Fight (Literally the most original thing you can ask for since you can shape your own Alphys here, doesn't have to be annoying, doesn't have to be too much like Undyne, just awesome.)

Possible ideas:
- make a lizard tail like armor weapon, mace or something idk, either way just a design idea

- make a half armor including nature parts or go fully metal, scientist armor (either of these I would do if I were you)

- you can make a little "fight attack break" referencing Mettaton's and Napstablook's one in battle where Alphys would just take out a manga and passionately finish what came to her mind quickly

- she could be one eye blind, I dont see why everyone only thinks of sans in any more radical design choices (could hide it with a leaf if you go for the nature armor idea)

- Grillby Fight (I know, there is one in Unitale, whatever- Make your uhh.. own.)

- Muffet Fight (now this definitely isn't canon, but if I made my Underswap I think this would be pretty cool, just a pacifist training session, could even include swap Sans.)

- Pacifist Papyrus Fight (It's a swap, many thought he can have a Pacifist Fight and why not? He'd stop us for our own safety.)

- Muffet Date (I am indeed serious. Yoda suggested it, he said: "dating Muffet fun could be considered. original idea, that is."
Seriously though, why not? No one ever made it, isn't it an original idea? Everyone wanna follow the pattern and make Undyne and Papyrus swap characters have dates. Why? Just do something new.)