Well I have barely seen Geno's name come up in a recent game, but then popularity and # of games doesn't matter much to Sakurai, but if Sakurai is going to have a character represent a series, chances are he'll be going for one that doesn't play second fiddle. Geno is second fiddle. One of several in SMRPG. Is he the most liked? Maybe. Does he have a moveset? Absolutely. Is he the clear candidate to represent SMRPG? No!
Count Bleck & Dimentio on the other hand are not only major characters, but they are recognizable. Waluigi possibly even more recognizable than the aforementioned duo thanks to the spin off sports games and the Mario party series. Loved Waluigi's Island in Mario Party 3. I think that would make an adequate stage for him. Assuming hes in...which is far more likely than Geno getting in.
Lol wut? Count Bleck and Dimentio more recognizable than Geno? I think not. Geno's had over ten years to build an often underestimated fanbase. Count Bleck and Dimentio have had a few months, and even after that they don't have much to show for it. Not to mention that Super Mario RPG is far more influential and overall a more important and memorable game than Super Paper Mario is or will be. I have yet to hear someone say SMRPG was a bad game, but already I get mixed reports about SPM.
And Waluigi HAS been in more games, but think about what he's actually done. NOTHING. The only game where he's done anything on his own was DDR Mario Mix, a game that was itself filler. He didn't even play that big a part, either. Also, his fanbase is tiny, and no one's expressed any strong desire for his entry. To say his chances are better than Geno's is, to be blunt, stupid.
Out of all the potential new Mario characters, Geno's gotten the most attention, both from fans and Sakurai himself. and since Dr. Mario's pretty much gone, you can't just say that there'll just be four Mario characters. The one and only threat to Geno's chances is Bowser Jr., and even that's unlikely.