Personally, I’d think it’d have to be a representation of the player characters or a notable NPC like Morden.
I actually have a notepad of a hypothetical move set for the Peacekeeper and AO protagonist. I feel like the PK could cycle through the seven Source Magics occasionally like Shulk’s Monado Arts, while the protagonist of AO could represent a four-way Savant.
peacekeeper has too many varied abilities for a fighting game like smash imo (3 of 5 different possible magics plus mutations) and everyones peacekeeper is different so it’d be harder to accurately design him
hmm well we don’t know enough ab cursebeard and durza’s fighting styles but in comparison trigno would be easy to make a moveset for (plus he’s a fav of mine, and pretty well known) someone more recent like iris or morden could work equally well tho like you said