Super Smash Bros. is a huge franchise, so if the games would ever have to be rebooted with a more concise roster, most franchises would have to get downscaled. In some cases, only a single character would be left to represent a franchise, and that's where we are today. You can see the thumbnail, so no more dallying.
First up is Fox. He would have some reworking to do, since his moves were mostly improvised for the original game. In this reboot, he would mix two fighting styles: for some attacks, he would fight like an Arwing, but for others, he would simply employ Krystal's staff. You might be confused as to why I say fighting like an Arwing, but really it just means that he would draw moves from the on-rails shooter. This means things like changing his blaster to shoot green blasts to match the ship (along with the ability to charge shots) and replacing his side special with using a bomb wisely, and he would have a jetpack with retractable wings. This would manifest itself in his animations for dashing and double-jumping, his dash attack spraying foes with the thrust, his dodge being a barrel roll, and his recovery being a boost upwards. The other part of his fighting style would employ Krystal's Staff, with using it physically filling out tilts and aerials, fire projectiles (albeit at a shorter range) as a forward smash, ground quakes as a down smash, and ice projection as a down special. His final smash would also be reworked to put him in an Arwing instead of a Landmaster.
Honestly, Super Smash Bros. for Wii U nailed the idea of a Star Fox stage: putting you right in the action of a Star Fox dogfight. This concept would be revisited to give Fox a worthy home stage.
The Mother trilogy is held as a trio of classic JRPGs, and of these, Earthbound stands out as being the only of the games to release outside of Japan (outside of Virtual Console), and Ness stands out by being one of the twelve characters in the roster for the original game. Since he uses moves from his home game and lacks more recent source material, there's no reason to update his fighting style. To be honest, I have no ideas for a stage for him, having never played the game myself. If any of you have an idea, I'll put it here if you can get it within the week.
Another character that would go unchanged would be Kirby, despite the heaps of new source material that came out since his debut in the original game. The reason for this would be out of respect for the creator of the Kirby series and Super Smash Bros. mastermind Masahiro Sakurai. His contribution to the series has been immense, so if given the keys to the franchise, I would respect his representation of his own character.
As a part of this respect, Kirby's stage would actually be a remake of a previous stage, something that I have been avoiding for this series. More specifically, I would remake the Fountain of Dreams from Melee, mostly due to its popularity in competitive play.
Pit would also be unchanged, but for a mix of reasons. Like Kirby, he would be unchanged out of respect for the man who brought him into the modern era: Masahiro Sakurai. Like Ness, he hasn't seen an update to his source material since Smash 4. Overall, there's little reason to update his move set.
For the same reason as Kirby, I decided to have Pit's stage be a remake of an older one, namely Palutena's Temple. It would be the same stage we all know and love; the only change would be revamping some of the easter egg dialogues for revamped characters and adding new ones for the new characters.
That just about wraps up this post. While the new Smash Bros. game probably won't be a reboot, and even if it was, it wouldn't be anything resembling this, but wondering about where the series could go drums up the excitement for the next iteration of a beloved franchise.
Read the previous parts:
Part 2 - Mario Adjacent Franchises