I think it's true that we won't just keep getting more and more, but we are by no means scraping the bottom of the barrel right now. I fully expect the roster of this game to be between 50 and 55.
The only thing people seem to underestimate is Sakurai's ambition. The man is a fuckin dynamo.
There are tons of great characters left to use, but we have 0 'mario/link' level of characters left. No main protagonist that has been in nintendo games since the NES or SNES and persisted through the years. Either the franchises died out in the 80s/90s or they're new. No more 'obvious' additions. Now it's just villains, or characters that are new/have only been in 1-2 titles.