To be honest
While I'm sure there are ways to make a Hypothetical Smash 6 more enticing through the gameplay, it's a very Huge gamble on that front
Many People do like the Gameplay of Ultimate already. While it's not perfect, it has a lot of polish and enjoyability. in regards to what potential gameplay changes we could get
-Changing it to a 3D Based game, Whether it be an Arena fighter or something else, would be a Giant shift, one that may or may not Pay off. Arena Fighters, despite how much flak they get, are very successful style of gameplay. I know people also like 3D Movement so it could be a change that could pay off, but would require alot of work, Since you're pretty much overhauling the game and also the characters as well to make them work in this engine
-Keeping the same gameplay, but revamping the movesets of the veterans, I don't know would have that much of an appeal to the main audience. I'm unsure if the Casuals really do pay attention to what moves certain characters have to that extent. in any case, I really think people overexaggerate how many characters really need changes to their moveset to such a level. Most might need One new move added in there or something, not a drastic shift in their gameplay. I think Custom Moves (that operate like Actual new moves) would work better in this instance since it could add something new to the Veterans, but keep their gameplay intact as well. I think people just want some fresh stuff added to the old characters, but there is no need for drastic reshift for most of them anyway
-Making the Gameplay Customizable would be interesting. More than just the Custom Moves, perhaps it could be interesting if the characters in Smash were instead used as some sort of canvas for people to make their own style of gameplay, whether it be through Moves, Colors, Costumes, Damage, Speed, Weight, etc. This has a problem of making the Gameplay not as consistent and tuned for competitive, and could throw balancing out of whack. Also if 3rd parties are gonna be a big part of Nintendo, there could be issues in how customizable certain characters would be allowed to be, especially looking at how certain 3rd parties don't have the best alts.
-Making the Game more "Hyper Focused". This could go hand in hand with the above ideas, but the gist of this one is making the gameplay as crazh as possible in terms of how it works. Stuff like having MVC type Combos, Air dashes, Super moves, Combo Breakers, Move canceling mechanics, Some other sorts of crazy mechanics that would make the Smash system more Hyper tuned. Unlike the above ideas I'm unsure if this needs as many sacrfices to the overall game, since it's still using the Current Smash system as a template, but just overtuning it to very exciting levels. If you also have the game system have some unbalanced type stuff, perhaps not as much balancing needs to be done on characters, and when working with a Large roster like Smash, that will definitely save time
-Turning Smash into a Traditional Fighter. Not really the best idea since Smash not being traditional is a huge part of it's appeal. It would be interesting, but would probably be a bad idea in the end
-Shifting Smash into Another Genre. I'm unsure of what genre they could shift to without alienating alot of the playerbase, and even without that alot of Sacrfices would need to be made depending on what Genre they decide to Switch to. Plus I think Smash would be unwise to give up it's Monopoly on the Platformer Fighter genre currently (this also applies to the Traditional Fighter idea)
If they really want to try to put an Entire different type of Gameplay in Smash 6, that could work in making it more standout from Smash Ultimate, but it's a huge gamble since there be alot of sacrifices that need to be made to develop the new game systems, and you run the risk of the New gameplay style not being as good as the System we have now
If you stay with the Current system though, it will be really hard trying to different itself from Smash Ultimate that would make people think the sacrifices made would be worth it. Revamping Movesets I think WOULD NOT be worth it at all, and would just cause other potential problems as well. I think the main thing Smash could do to make the Gameplay a main selling point is make it more customizable in Mechanics, Character attributes, moves, and cosmetics, and perhaps other miscellaneous things
I think really, In order for Smash 6 to really succeed, it's gonna have to have Alot more New stuff in terms of the Roster itself, and the Content within the game. I don't think, as an example, keeping a Majority Veteran based roster would work out to well, whether you keep the gameplay system or throw it out completely, since both approaches have their Faults that could make the risks too high