I really have to wonder. Assuming Smash 4 makes no cuts, we get Mewtwo and Roy back, as well as Ridley, K. Rool, Dixie Kong, Little Mac, and a Mario newcomer (one of Toad/Paper Mario/Bowser Jr./Waluigi) are added in. Then there basically won't be any characters left with large passionate fanbases for Smash 5 aside from possibly other Mario characters.
I think most of the characters that became big during the pre-Smash 4 period (Palutena, Shulk, Chrom) will not be highly requested for Smash 5 if they fail to make it in for Smash 4.
Pretty much this has been my thoughts for a while. Assuming we get little to no cuts, Mewtwo and Roy back, Ridley, K. Rool, maybe Dixie Kong (Though she's not really requested in the first place), Little Mac, a Mario Newcomer, and Isaac, we basically got a lot of major Nintendo characters. Really as well? That's only five newcomers. Let's just throw in Pac-Man and Mii there if the rumor is anything to go off of. To be extra nice, how about a Retro and maybe a "WTF"? There's most, if not all, of your newcomers. If you want, maybe replace Roy with Chrom or something.
Smash has a choice if they choose to expand the roster, and they can mix them, but...
A. Go for smaller series to add characters to - To this day some of the Smaller series have character requests. Krystal, Goroh, Masked Man, etc. I've said this before but based on other fighting games and Smash, I don't think they should be ignored considering there would still be wants. A lot of said series are still expanding or are in consideration anyways, and you can't really ***** Mario/Zelda/Pokemon forever. The problem still in this is not only is it hard to see requests on these, but it's hard to think up on these despite the pool being so small. It's basically more of a merit on importance than on popularity. (Not that most of the characters we want don't have that or anything)
B. Go for smaller series that don't have a rep - There's a lot of these from Nintendo and it's already a direction they're leaning in.
There are other things as well to this. After the huge Brawl poll that NSider had for the U.S., where else can they go with seeing character popularity? Will most of the same producers stay on-board? I mean, Sakurai is only 43, but if I recall right, one of the main artists has cancer (Or this could be for a different game). For the most part, it'll be even less fanbases than with Smash 4.
This really does bring a lot of thoughts to the table.