Super Smash Brothers Ultimate - Last fighter coming to ultimate. Time to get ready for the welcome party. Who do you think Sakurai will reveal? 💌

I think we can all agree, "anyone but another Fire Emblem character."

Jokes aside for one moment, buffed Samus is best Samus.
Ok, enough hornyposting, I say it's going to be either someone from Pokemon or Fire Emblem.
I'm not particularly familiar with those series but seeing the pattern for the last few years I can say that those are the most probable outcomes.