Oh man, if I were the Smash director, 99% of the fans here would hate me and stop playing Smash Bros lol.
Assuming I was only the directer of a new game, and all of the previous games still existed:
First thing I'd do is take characters that seem too similar to their counterparts and make them skinswaps -
,
,
,
,
(So that's 5 cuts to the roster size)
Then, I'd make sure that each original (SSB64) franchise gets representation: So
gets Black Shadow,
gets Porky
I'd add Dixie Kong, Geno, and Waluigi due to popularity
Instead of trying to add a bunch more 3rd party franchises, I'd expand existing ones. I'd add Dr Eggman, Dr Wiley, Sephiroth
For my most unpopular part, I would make some cuts:
,
For stages, I would actually cut many. I'd try to keep all of the popular ones (Fountain, Corneria, Battlefield, Final D, Kirby Tree stage etc) but I'd get rid of the scrolling stages (maybe have like 1), all of the ginormous stages, and stages with really annoying hazards.
For the stages I add, I would add some fun ones that wouldn't be viable for competitive, but I'd make an effort to add at least 5 new stages that are "basic" (for Comp) but that make up for it with amazing visuals and music.
For Modes:
I would add a story mode that has In game engine cutscenes to save on time, but it would be pretty lenghty and similar to Subspace in epicness
I would try to give players a lot more control of items, and with items like assist trophies and pokemon, allow players to decide what characters/items can spawn from these.
I think that's it.