I'm going to start by saying that I don't think the core gameplay of Smash needs any serious retooling or expanding upon. Sure, it could be improved to better suit players (in an ideal world, there would be a game speed option to allow more fast-paced matches like those found in Melee), but I don't think we need things like aerial smash attacks or universal Shield Bs. Smash owes at least some of its success to its simplicity, and adding more buttons to the fray hampers that. I'm not saying we can't have wavedashing or L-cancelling or things like that - I'm just saying that we don't need more dedicated attack buttons or things of the like.
Pokémon owes at least some of its success to its simplicity, yet you don't exactly hear the end of fans (at least before Arceus) that the series needs "real change" to its gameplay instead of just Megas, Z-Moves, and Dmaxing. Personally, I think a super meter mechanic alone would be akin to Z-moves, albeit kept with the series for the future rather than scrapped in the next game. Even Kirby is loved for its simplicity and Star Allies was bashed for feeling too much like the previous games.
I think we could do with "updating" some of the older fighters and bringing them up to modern standards. Now, this is a slippery slope. Some fan reworks I've seen solely focus on incorporating references just to have them, and that just won't do. Fighters should strive to incorporate their source material to some degree, but giving Mario access to every power-up under the sun or giving characters extremely complex mechanics just to bring them closer to their source isn't the way to go about it. (Not trying to diss things like Inkling's ink mechanic or Arsene - Those are fine. I'm talking about things like Kazuya's plethora of complex attacks or even Terry having a Back Special).
- On that note, I actually like Wario Wario Wario's idea of having extra fanservice in the form of Custom Moves - Players who enjoy fighters' current kits can still enjoy them, while those who want more references get to have them (and more than they would have gotten if fighters were flatout reworked).
I'd personally go in the opposite direction and
simplify the newer characters down to classic 64/Melee newcomer standards. The existence of the overcomplex mechanics of the newer characters seems rather hypocritical given that one of Smash's goals is to be accessible and intuitive. I find a lot of the newer fighter frustrating to pick up because of this, whereas I happily leaped from character to character during the 64-Brawl years for casual play.
Now, on the topic of revealing fighters... I'm not sure how I'd go about retooling this aspect of Smash in a way that wouldn't annoy Smash players or Nintendo's corporate. On one hand, I understand where Wario Cubed is coming from regarding anxiety/room for more experimental characters/etc., but I don't think I like the idea of "everybody's just in three trailers". If it were up to me, it'd be handled more like Brawl's Dojo, with fighters being revealed every other week or something along those lines. Again, not really sure what I'd do here, but Wario Cubed's approach holds promise. (I'd miss the CGI trailers, though.)
On the topic of fighters, I think we could do with some more obscure/weird/curveball picks. Probably not to the same extent as something like NASB or Multiversus (gotta placate Nintendo's income somehow), but we could have a few more than we do.
I think we need obscure picks
with the big CGI reveals. Smash needs to turn those C-List stars into A-Listers with those trailers.
I suppose the only problem with them is that CGI reveals would be rather overdone for more simplistic 64/Melee-style characters, but I don't really mind that issue.
Add more single-player content. Sure, I don't hate World of Light or anything, but it's not enough. Adding more single-player stuff doesn't necessarily equal Subspace 2 or anything, but we could do with some more modes. (I will defend Smash Tour until the day I die.)
I don't think we need another iteration of a Subspace-style mode given how much development that took, but I don't want another single-player mode to feel as unfinished as Melee's Adventure. We need a compromise of Subspace and Melee Adventure somehow, but I just don't know what.
Finally, I would say that Smash could operate really well with a "free content update" system, but Nintendo fans HATE that sort of thing (and to be fair, Nintendo does it really poorly), so it'd have to be done really well in order to stop mass rioting.
Game-as-a-service just doesn't work at all for the vast majority of games. At least Epic is committed to never having to make a sequel for Fortnite, but then we have ActiBlizz who wants to sell a glorified update as a sequel to Overwatch.
And to be honest, even the "ideal" scenario of the next Smash staying where it is 'til the end of time is awful, since it would exacerbate Smash's borderline stagnation with its core gameplay.