I will not be surprised if Sakurai does return, I will also not shed any tears if he leaves. He has left a template for how the series expands and there are logical areas where we can easily expand.
1. More characters: The most obvious but easily the most difficult because Smash is running out of main franchise reps. Mario really has Toad (or Captain Toad whatever) as the most noteworthy character not in the games currently. DK could pull Dixie or heck even K Rool (although I doubt it sorry Mantyke) Zelda has had 5 total for 3 games now (if you count Zelda/Sheik as always separate) and who else is there really besides perhaps Tetra/Toon Zelda and Impa? Pokemon always gets at least one more and is the easiest franchise to expand. Kirby is dead in the water, there are no continuing characters left that they could pull. Metroid franchises only hope is something crazy like Fusion Suit Samus (the trolling would be strong in this one), since Samus can clearly be the only rep. Fox can have Krystal and bring in Wolf again. Fire Emblem can expand to someone not swords since they are running out there, bringing back Roy in our dreams. But really after Shulk and Mii Sword we have a wealth of sword fighters and there isn't very much more.
I don't really see certain franchises getting new characters as well: Animal Crossing, Wii Fit, Pikmin, F-Zero (unless new game), Kid Icarus (if anything bye Dark Pit). There isn't much that their main rep doesn't do already. Plus since Melee we've had our requisite retro character, who else is there? The Famicon Disk thing, Takamaru???
So if every old character (not Pichu) came back: Roy, Ice Climbers, Wolf, Lucas, Ivysaur, and Squirtle what other new franchises could be added, and lets face it new franchises are always added.
Well we have: Splatoon, Custom Robo, Star Tropics, Nintendo/Advance Wars, Excitebike, Wave Race, Golden Sun, Daigasso Band Brothers, Drill Dozer, Sin and Punishment, Starfy, Dillon's Rolling Western
But really that is digging a bit deep, and while there is always some love for an old franchise or two there is pressure on the newer as well and it's a bit frightening when we don't see much else making it in the same way Animal Crossing and Pikmin were always contenders for Brawl.