Once again I want to bring up the possibility of keeping over 70% of the character roster retained, but taking big axes and cleavers to the stage roster instead.
I'll ask the armchair developers here--would a drastically smaller stage count positively affect the number of veteran characters returning, or would it be negligible in terms of time and resources? I'm willing to sacrifice stage variety for character variety to flourish further.
It's likely the way they allocate budget and potentially have sub-teams working of different aspects of the game makes this an oversimplification, but in every game they could've theoretically cut content other than characters and funnelled that back into character development.
And they do cut some things that draw resources wherein that time/budget could be spent elsewhere, like the removal of target tests, all-star, trophies, etc.
So if anything were cut to redistribute the budget elsewhere, it probably wouldn't be stage count. After characters, stages are not only one of the bigger draws/focuses of promotion, but an aspect central to the game. Stages are actually important. Unlike some modes which are neat but ancillary.
They're also something that can be sold as DLC. They weren't in Ultimate (separately from fighters), but it's possible they bring that back. And that could probably be pretty lucrative considering if they sell them for even half the price of a fighter, it's certainly less than half the work than to create a fighter pack.
Plus I think you would be disappointed in how many stages it would take to equal one fighter in terms of time and resources.