In my ideal world they'd make a Smash Bros designed to last for the next decade. As in a game with a foundation so rock-solid that the support for it transcends console generations, allowing the devs to continuously build upon it.
Start with the next game being a soft refresh that trims the fat of the roster, updates the veterans, overhauls egregiously bad movesets, have an all-new stage list, complete with additional modes and an in-depth story mode like Subspace (with actual dialogue); if there was any time to go hard on a story mode, it'd be when the roster was as small as ever.
Then periodically add DLC characters, skins, and stages throughout the console's lifespan, in addition to adding stuff that we've never really seen added post-launch like items, assist trophies, story content, and the like. Ideally the content is consistent and lasts longer than Ultimate's content cycle, maybe going for a (NON-OVERLY-PREDATORY) live service format where a bundle of new small stuff is added each month like a handful of skins.
Then when the Switch 3 rolls around, and since the intent for this Smash was to be cross-generation, you can transfer your save data to the new console while the game itself receives a hefty number of newcomers, modes, and stages for the new console.
I believe this is the way to achieving 100 characters in a Smash game of any sort, while making sure that vets don't feel outdated and the whole experience feels fresh too.