Well since everyone is in the mood might as well do it, forgive for the length and just be careful for spoilers if you wanted to play any. Just going to put together Luminary and Erdrick.
Geno: SMRPG is a series that was my actual introduction to RPGs when I was a kid (after a failed attempt at one of the NES DQ as I didnt understood the game), and is a game that I enjoyed thoroughly and still holds up. That said Geno is probably my least wanted here just because, I like everyone else more and the I find that the other series can bring a lot of more of neat stuff. And after replaying SMRPG recently, I found Bowser and Mallow more endearing than him.
Sora: KH is a series that I enjoy due to the crazy concept and mishmash of Nomura's designs and Disney. While its a series that makes little sense storywise, gameplay is fun and the series is quite a ride that I'm looking forward to playing 3 soon. Sora's abilities and playful personality would make him a great option, plus KH music and worlds are simply great. Overall Sora is a fun character to imagine in a game like Smash so he is my 3rd option.
Crono: If SMRPG was a game that properly introduced me to JRPGs, CT is the game that made me feel in love with them. Great music, great characters, enganging storyline, cool twists, great design, great graphics at the time of the SNES that still look great today... One cant praise this game enough. Crono is my 4th choice as while CT is a great game, I'm just feeling the others more.
Sephiroth: FFVII was my first FF and the impression that Sephiroth makes when you first encounter him is wow, he just casually murdered everyone that was in the Shinra building and you get lucky that you were imprisoned. His theme is pretty much one of the best in VII, and just the presence and actions he does in FFVII, not only regarding killing Aeris, but basically messing with Cloud's mind during a grand part of the game makes it a great hero-villain relationship. He is my 2nd choice, after all we got Cloud in the game and his katana would make a great moveset.
Erdrick/Luminary: DQ is a series that while the MC don't particularly shine due to being a blank state for the players, but these blank states help us to immerse ourselves fully into the world of DQ and experience unforgettable adventures. The world is filled with cool monsters, happy stories, sad stories, epic battles, great locales, memorable tunes (especially older games). The hero is also great due the amount of abilities, both magical and physical, and he would fit great with other heroes already in the game like Link and Cloud. He is my 1st choice.
Slime: Just like Erdrick, he is part of what makes DQ great. We have a few mooks in the game and he would fit with them, but at this point if you want to represent a series like DQ I would prefer a hero vastly. He would be my 5th choice.
Overall, I would be happy if any of these picks get in.