Regardless of her fascination with the dark arts and the eldritch, Ashley is probably the most neutral person you will ever meet. Her first appearance was the closest to being ''villainous'' (not that Orbulon is that nice in the first place) what with having no compunctions against tossing sentient beings into her potions, but ever since she has just kept to herself, neither helping nor hindering anyone. Though it doesn't hurt that the ''demonic'' creatures and objects she hangs out with are honestly pretty nice. That skull book thing in Smooth Moves pretty much just helped complete her spell, with no strings attached! So I'd imagine she wouldn't mind being friendly with anyone that is nice to her. I think Dr. Coyle is far more likely for a female antagonist (though even she isn't that evil, just misguided).
vs.
or
, with Ridley taunting them over their dead parents, noting a look in their eyes he sees in Samus.
&
quietly reflecting on the latter's loss.
with, well any other Zelda character just for the bafflement of seeing different versions of himself, Zelda and Ganon.
&
pontificating about the meaning of the fight.
(or even better, Rex & Pyra/Mythra) &
debating the merits of godlihood.
could join too.
I've already mentioned Ashley having possible synergy with Dark Pit, Bayonetta, Ridley and Luigi, but I realized Mewtwo could work as well, two beings who prefer to spend their time away from others for their own reasons. Though their first meeting would probably have her confuse him for an alien like Orbulon, causing his temper and powers to flare momentarily and giving her something of a taste of her own medicine.
Falco's part of Star Fox, this gives him access to Star Fox equipment and training, ergo plausible clone of Fox, ergo easy inclusion.