On the topic of saving characters for the next game...When are we getting another Smash?
Not only is this the second year of a, no pun-intended, "Smash hit" console, but this game is based on packing in as much content as possible. It's being designed with longevity in mind considering the focus on solid, competitively viable gameplay. There are more Assist Trophies than ever. Every character in the series is back. A lot of stages are returning when they hadn't before.
In addition, this is the 2nd (technically 3rd) straight Smash game that Sakurai has worked on without being on another project. That's if you count Wii U and 3DS as two separate games.
Now, I can't say this with total certainty but it seems to me that this game is meant for the long haul. This isn't really meant to be replaced with a true successor for some time. Unless, of course, they're just going to try and reboot Smash (bad idea in my humble opinion).
If the Switch, as a console, has a healthy lifespan, we're not going to see a new Smash for a loooooooong time. It will be at least until there's a new version or successor to the Switch. In between this Smash and the next, we could absolutely see ARMS 2 + 3, a few more new IP, another Xenoblade, another Generation or two of Pokemon, etc... There is plenty that would happen in between this Smash and the next.
Are they going to add literally every character that makes sense to be added right now? Eh, I'd be surprised if that happened. However, make no mistake, I cannot see how DLC wouldn't be implemented for this game in the form of Spring Man at the very least. In terms of "series representation" in the forms of the characters themselves, DLC will be the perfect way to tie up any loose ends that characters in between the project plan and now are a part of.
After that, when we have newer games or newer series, reserve those characters for the next Smash. It is clear that Ultimate is meant to be a focus for Nintendo for an extended period of time after release. I wouldn't even be surprised to see a yearly Invitational at E3 considering the emphasis they not only placed on the event itself, but the coverage of competitive techniques in the Direct as well as some of the gameplay footage from the Treehouse.
To close, any remaining characters after this game are simply not going to stack up to any veteran. Ridley definitely got a pop out of fans with his reveal, but the true cheers in a ton of reaction videos (such as the one in NYC) came from the returning veterans like Ice Climbers, Pokemon Trainer, and Snake. There's a value and focus on veterans and Smash history. Because of this, I personally feel Smash is going to be on an extended hiatus. Yes, it's a money maker and it might not make complete sense to not release a new Smash, but this is the biggest one yet and for good reason.
This game will continue to make Smash history in a big way. They're very likely going to go all out, or at least "as all-out as they can".
That's my humble opinion anyway.