I think considering Mario has four good potential characters that all have fairly high popularity, recognizability, and potential, chances are pretty good we'll get one of them. Sure, none are at the level of the four we already have, but at this point they're at about the same level as most of the other potential additions, plus, just by the fact that they're Mario characters, they'd already have quite a large potential audience familiar with them.
I don't think getting a fifth Mario character is as likely as getting a fifth Pokemon character, (it could be possible Sakurai would think adding one or two DK characters more than compensates for a main-series Mario character, though I'm not convinced he has that mentality), but I still think getting one is a probability. I personally think the chances are higher than getting a fifth Zelda character.
Plus people who bring up "we shouldn't get more than four Mario characters" seem to have forgotten we've already gotten more than four Mario characters.