Personally, I just don't get the rush to cram in as many new franchises on the roster as possible before Ultimate ends. A few maybe, but not for every single slot.
Yes, I understand that this is supposed to be the "ultimate" Smash, but people are starting to act like the number of franchises added to the crossover is the only thing that defines this game as "ultimate". Part of the reason why I've liked Ultimate's lineup so much is because it balances the new and old. You've got Splatoon, Castlevania, Persona, and Dragon Quest, but you also got to see the Mario, DK, Metroid, Pokemon, and Animal Crossing expand beyond what they used to be.
If this was the direction Ultimate took from the very beginning, yeah, we'd probably have a ton of new franchises, but it would be at the expense of fleshing out the existing universes. They're already represented, so why bother with them? They won't contribute to the crossover element.