Coming in all the sudden~
The way I see it, in terms of probablity, it goes Paper Mario>Bowser Jr.>Geno in Likely to least likely order. Why you ask? Well, it's because of several reasons.
First off, why Paper Mario is ahead of Bowser Jr. This reason is relly quite simple, and is basically the only thing that seperates them, who they are based off of. Paper Mario is a second Mario. Bowser Jr. is a second Bowser. Of course one is much more literal than the other, as B Jr. is indeed not really Bowser, while PM is indeed Mario. But despite that technicality, the little guy is really just his father but young. Heck, Baby Bowser looks just like him. The apperance and name are just to simmilar for them not to be considered basically the same. But does that hurt him? Not really. We've had characters that are close to each other before, and as I said, I think Paper Mario is more likely, and he literally is a second version of Mario. It's just that we'd get a second version of Mario before we'd get a second version of Bowser.
As for the case of Geno, the reason I put him lower than all of them is because his status and third party nature both hold him way to far back. He showed up in one game, and had one cameo, but that's about it. Paper Mario stared in three games, and had a cameo in Melee. Bowser Jr. showed up as the final boss in two games, had playable roles in many other games, and also showed up in Galaxy. Popularity isn't really much of a factor here either, as the other two are both very popular, by the virtue of the fact that they had so many games. Uniqueness in abilities is also around the same for all of them. (Really, TC, the only way someone should really max out on something like that is if they are like the ICs or PT) In any case, Geno is outclassed here many times over, and in addition, is a third party character. Thus, he is much less likely than the other two canidates. =/