One thing to consider if they're supposedly going for new audiences with the DLC packs, so there's no point picking something that's either doubling up on an already present demographic or something that won't bring in a new and sizable demographic.
>Joker: liked both overseas and Japan, anime/other spin-offs help exposure, "pop JRPG"/non-gamer appeal (that sounds kind of ridiculous but plenty of people consider themselves Persona fans and haven't ever played P3/4/5)
>Hero: cultural icon so he crosses age boundaries, also serves the purpose of promoting SE's stuff
>Banjo: imo doesn't bring much new, even if I love 'em. Older Western fans? Just a general crowd pleaser?
>Terry: huge arcade history, FGC representation, reasonably popular in Mexico/China from what I've heard
They could go for age demographics, genres, regions, etc. I don't think a character like Kosmos (known more for crossovers than her own series) or Tales (another RPG?) or even Touhou (another Japanese cultural phenomenon) necessarily covers anything not already there. Capcom have a few viable contenders assuming RE/DMC/MonHun fans aren't already in Smash because of Ryu/MegaMan. A big Western company could attract a lot of attention (Rockstar, Bethesda, Blizzard, Valve) but we already know dealing with overseas companies adds a layer of complexity to negotiations.
The demographic might end up being a total curveball (like "new Nintendo pick" to attract younger gamers, or something totally bizarre like "Wii U fans who haven't migrated over yet") but it'll make sense in retrospect when considered this way.
tl;dr fighter 5 will get vaguely defined groups of people to open their wallets