Honestly, I have a hard time even thinking about characters that are region exclusive that could be included. It seems like even the most impossibly niche stuff gets localized these days, so it could only happen if it's a deep cut from the past like Doshin or something.
I don't have any attachment to this rule either way, and would find cool to see some of these weird picks should they happen, but I still believe that as DLC they are almost impossible. Characters from popular, yet niche (by comparison) series like Persona, Dragon Quest and Fatal Fury/KoF already evoke knee-jerk responses by the community, imagine someone region-exclusive from a 20+ years old game.