In Regards to Everyone is here being a thing for future Smash games, I'd assume by the time we get Smash 7 or 8, That AI might be developed enough to the point that the can expedite the Process of Development, or make it faster and easier on the manpower required, that or any other type of technological advancements in Software development
On the subject of new mechanics, how about something like this: Assist Spirits. They would be a meter style resource with several charges that you could build up over the course of a match. Expending a charge causes the Spirit to come in and do an assist type move. Building the meter fully allows you to use a Super Assist, which is basically a super, but for your assist character (I have a way with words, I know).
Lets use Waluigi as an example. Waluigi's Assist has him appear in front of the player, and swing a tennis racket. This attack is a reflector, but otherwise is fairly unremarkable. His Super Assist has him kick a soccer ball at the nearest opponent. If it hits, they are wrapped in purple vines, stunning them. Like with all stun mechanics, this is more effective at higher percents.
Could be a fun way to add assists and supers at the same time.
I actually had thought about there being some type of Assist System in the game to make Assist Trophy characters more prominent, especially since It can show of the newfound power of the Switch 2, and it allows more Characters to be represented in the gameplay for the crossover. but honestly, I think even the idea of having to summon a Character is too complex already for Casuals, nevermind adding some type of Super meters and moves tied to it. People even have problems with Tagging characters in, which might be part of the reason 1v1 Fighters outsell Tag Fighters
Also Assists I think are at their best when they are allowed to move around in this platformer fighting game. I don't think Assists in Smash can work like other Fighting games just apparing in front or behind the player
I think the best thing to do with an Assist Mechanic would just make it so the Assist Character is always paired with the Main Playable character. As an Example, You can play as Sonic Regularly, or you can play as Sonic & Knuckles, where Knuckles will follow along with you and has a Set number of Attacks you can control him to do. However you Share stocks so if one of you dies, that's a lost stock for both
As someone who was (and still to some extent is) a proponent of Ultimate Deluxe, the factor that attracted most people to the concept was pretty simple: people aren't sure if there will ever be a Smash game that could follow up on Ultimate.
Most people can accept a Smash game with fewer characters and not every veteran, but most people question what would need to be done for a new Smash game to not feel like a step backward from where Ultimate was. I think it can be done, but what that game would look like and how ambitious Sakurai would need to be are still kind of up in the air
We're Smash fans, so we'll at least be open to whatever comes. I don't know about what the other 29.9 million people who bought Ultimate would be receptive to.
If you go by Youtube Views and also by trends on what sells the Most, it Would definitely be some type of Big Single Player mode with Cutscenes and an actual Story. The World of Light Announcement for Ultimate was one of the Highest viewed for Smash Ultimate, and the Subspace emissary has alot of Views for it's Cutscenes
In Any Case, I do think Sakurai and the Team will try to achieve Everyone is Here again, in addition to new additions. Yes Everyone is Here almost didn't happen, and even acknowledged as recent as a year ago that it would be difficult to do again and would really need to think if the Next Smash game is worth upsetting some fans
But because they did everyone is Here, it became the Best Selling Fighting game of all time, on the one of the Best Selling Consoles of all time. They allowed the Extra Development needed for both TOTK and Mario Wonder, so I Imagine for the successors to the Big 3 games on the Switch (Mario Kart 8 Deluxe, Animal Crossing New Horizons, and Smash Ultimate), they'd be willing to do the same, to sell the Next Console to people that already have Switches, and even people that don't, I can already imagine if the Next Smash had all of this
-Every Characters comes back...AGAIN!
-100+ Character Roster
-Story Mode
-Rollback Netcode