While I realistically don't think Ninten will be added to Smash, the huge Mother fan inside of me really wants him to complete the protagonist trio. I would love to have another Mother rep, especially Ninten, but even with the love Nintendo has finally been giving the series, I just don't think EarthBound Beginnings is nearly as loved by people as much as EarthBound or Mother 3. It's a real shame, because I think EB Beginnings is a fantastic game, especially for it's time, but since it isn't as polished as the other two games in the series it usually gets the cold shoulder from fans.
I don't like it when people say "THIS game is the best in the Mother series!" because I don't think there is a true best. All of them are different yet one in the same. It's hard to compare EarthBound, which is more nonlinear and a less serious, to Mother 3, which is more linear and is often more serious. The games are very alike, of course, but I don't think it's right to compare them as if there's a definitive game. EB Beginnings unfortunately suffers the most from this, since it's more primitive than the other two. I think part of the beauty of the series is how they're all unique in their own way, and because of that I think Ninten deserves a spot in Smash. I would love EarthBound Beginnings to finally get some of the spotlight.
Do I think it'll happen? Likely not. I think someone like Porky would be more realistic (who I'd also really like but I'd prefer Ninten) since he's in two of the games and serves as an antagonist in both, that is if another Mother character is considered at all. But would I want him? Yes, 100%. EB Beginnings is the most underrated game in the franchise and I think it'd be great to have Ninten added to the cast of characters. You have my support!