I don't think this theory is even close to the truth. The only characters that were considered but didn't make the cut, that we know about, were the Ice Climbers, and perhaps Chrom. However, there's no proof Sakurai ever did more than mentally consider Chrom. It's not likely Chrom even made it past the planning stage, considering how much TLC was put into Robin.
Also, Sakurai has said that he no longer obsesses over character inclusions beyond a certain level. His roster was picked from the very start; in 2012, I believe. Considerations were made, but nearly all the Brawl and Melee characters are back, in addition to a boatload of newcomers. Excepting obscure one-off characters, all the major Nintendo characters are already in the game, and most of the well-known sidekicks, villains, love interests, etc., are as well.
I think several of the assist trophy characters could have been considered on paper in 2012, but that's likely as far as they came. Bowser Jr. almost got cut, and Ice Climbers didn't make it. It doesn't sound like they had enough time to be fooling around with tons of additional characters with low priorities, if we lost one and nearly lost two.
To say nothing of Lucas or Wolf. Who knows how far they went. If you consider Charizard the spiritual successor to Pokemon Trainer, those are the only other Nintendo Brawl characters that didn't make it back. Sakurai said a while back that he hadn't cut anyone yet. Was that just a lie? Did he mean cut from Brawl, or cut from his Smash 4 master list of picks? We don't know.
If we ever get even one more character than Mewtwo, the only ones I think are likely to make the cut are the Ice Climbers. They already have near-complete data for them, as well as character models. They deserve the spot, I feel, over just about anyone else. That said, however, the difficulty in getting them to work on the 3DS and 8-player Smash might still be too great if the character isn't redone as a solo character of some kind... and seeing as how Ice Climber is an as-yet dead IP, anything beyond Mewtwo, if it ever happens, might just be a fresh face.
They're not going to "promote" a character like Ghirahim who's already in the game, that's for sure. Isaac? Possible but very unlikely, I feel.