Ideally I would like to see Bandana Waddle Dee but I doubt it. The thumbnail has already spoiled it with the flaming the smash brothers logo and the envelope. You will be able to play as the Smash Ball, the embodiment of the smash brothers, being able to use all the classic features like summoning trophies and launching coins. The gimmick will be that you can charge your final smash on your own without an external smash ball like Gogeta, and the final smash of the final character will be the final boss of the final the smash brothers, the master hand. Or maybe the Chorus Kids leak will finally become real now that nobody is talking about them anymore. Looking back on it, I do miss Smash 4 where it all seemed bigger than it was but it was still actually pretty big for its time. The directs all felt like they were greater than the last, always building up to something (MEWTWO) and really made you feel like this game had a lot of features leading up to release like with the 50 fact extravaganza. Even the Miis got advertising that basically represented real people, so maybe Tony Hawk could be the final character and they could add some more Mii classes after Miitopia's success.
No matter what happens, everyone's always got some kind of prediction and what I like about that is that I think that in my opinion widespread feeling of speculation has really kept the smash brothers going all this time.