This. It's already implemented in Smash Tour when you use all the fighters you collect in the board. Why not have another mode that does exactly this? Also stage hazard button would be nice too.

Oh and Smash team missed out in keeping boss mode from Brawl and letting the player controlling the GamePad control a Boss with large stamina and area effect moves versus 4-7 players.

I'd love a mode where the stages of the game have no blast zones, and you'd bounce back to the stage if you reached it. And there would be a percentage cap of like 100%, and when you reach it you explode.

And I'd love sudden death to be changed to where you permanently have 999%, you both get 5 stocks, there's no bombs, and fight until there's a winner. Or for the people who want to keep Sudden Death the same, you can make this as a mode (Both online and offline)

And a story mode like Brawl's.

120fps and a story

Ironically, Super Smash Flash 2 is going to have just that in the next update.

That would have been great.

Hard to build on S4 tbh maybe make the next game slightly faster but mechanically I really can't think of anything they can build on S4's system as the are so many different techs that can be employed. Any change would have to be character specific ones like improving Falco's laser and so on for example.

Story mode.

Different stages in for glory. Bonus if they have blast zones, variable elements, destructibles, running machines, water, etc.

Online cross-play. 120fps on home console (HH won't have the panels and this can't affect online because few have 120fps TVs).

Better techs.

Maybe something like pokkén's criticals.

Make ridley playable in Team battle as a Team a la Lightan and Ptx in Tastsunoko vs Capcom