I think I've said this before, but... I feel like restating it.
Geno is a Mario character. Mario is Nintendo's biggest franchise, so obviously that alone makes it deserving of a new character in Brawl. With Mario, Luigi, Peach, and Bowser the Mario series is already represented with the most famous, popular, deserving, etc. characters. Where would they go from here when looking for a new character? Some say Bowser Jr. He is afterall the Mario series' newest main villain. So what? He wasn't even on Sakurai's poll. He must not interest Sakurai much. Geno however, is mentioned four times. This shows Sakurai is interested in Geno, so when looking for another Mario character, he'd likely look to Geno.
That's atleast my reasoning on why Geno is likely to be included... Isn't this also what fellow Geno Supporters believe?
(I know I kind of bashed on Bowser Jr., but I support them both really. I don't see anything wrong with Nintendo's biggest franchise having 6 representatives. o_o [Though, I do support Geno more
])