He's the most requested newcomer, the most requested character is Mewtwo, but K. Rool comes in second place there.
Firstly, Diddy was already in Brawl and shoving Dixie on him will alienate just about everyone that played him. Second, it would be an insult to the both of them because they are both deserving characters in their own rights, shoving Dixie on Diddy is like saying that she wasn't good enough to make it on her own. And it doesn't fit with the series, if you really want to try and go there, they only ever paired up in one game, and in Tropical Freeze, they can't pair up at all. So if you say she's likelier than K. Rool because of that game, then you shouldn't say she'll be shoved on Diddy, because she wasn't in that game.
I'm not being blind, the only thing I'm doing is not following the sheep. The odds aren't really in their favor, people just think they're going to get in because of that "leak." Miss Fit already fulfills the role of a character for casual gamers, and Villager already fills the role of a generic avatar character, hell, he looks just like a Mii and his "everything and the kitchen sink" moveset makes Miis redundant as well. Unless Sakurai is going to make an exception for just Miis, they won't show up because they aren't unique enough. The only way I could see Miis happening is if they were some special option away from the roster where you could make a single, customized moveset for your Mii and only use it for certain situations.