Yeah, on the subject of new content for a new Smash game, the thing I really have to wonder about is the Single player content. World of Light had some good ideas, some of the Spirit fights and the Bosses are pretty fun to fight, but there isn't alot of replay-ability for yourself once you complete things.
I honestly think the times when it was replayable was when it had the platform modes, like Melee's Adventure Mode, the Race to the Finish part of Melee's Classic Mode, and Smash Run. Smash Run was very replayable just because of how fun it is to fight against enemies from different series while prepping up for that final showdown by going around a big map with platform elements while Adventure was fun for going through the different stages from different series, which World of Light sorta had this with some parts of the map and the dungeons.
I feel like they should really try to bring a platforming-based mode into the next Smash. Like some possibilities would be:
-A pseudo Metrovania-like mode where you fight against enemies from different series and have to follow a series of paths to the different bosses in a bit of a linear fashion, but enough choice to decide which end goal or boss you fight at the end. Something like Kirby and the Amazing Mirror.
-Smash Run, but expanded upon. Like for one:
Give an Options menu like Kirby Air Ride & City Trial to let you:
-Control on how much time the match is.
-What end game you'll do.
-Whether Events will happen or not.
-Which enemies you will face, etc.
New enemies from different series to fight against.
New powers to use, like an power to summon a specific Assist Trophy or Pokeball ally.
Some new events like an Event where you find and fight against certain Assist Trophy characters or Bosses.
Maybe possibly an additional map or two to play in that are more based on different game series in Smash.
-A straight-forward Adventure Mode like Melees with a bit of elements from Subspace and Smash Run, like doors, but also go full Mario Maker on the Stage Creator to allow players to not just make Custom Stages for Smash, but Custom Adventure Mode stages. All the stuff on Adventure Mode can be used in Stage Creator. And there are a big list of enemies from different series that you can place in Adventure mode stages. And for Adventure Mode, a main mode where you do the game's original Adventure Mode stages and a Mode where you do a series of created Adventure mode stages sorta like Mario Maker's 100 Mario Challenge.
Yeah, those are my dream content, but I do think the next Smash Bros.' single player should try to focus on platforming as the platforming-focused modes were the parts that I felt were really replayable.