If Ash gets Chimchar now... (which imo, is a very awesome twist, albeit predictable to a fault!) ... then I bet it Will evolve into it's final stage, just like Charizard. Paul and Ash will have to battle in the league and Ash will beat Paul, with (the newly evolved) Monferno, because the monkey will be obedient. Unlike Charizard was.
Well, anyways, after that, Chimchar/Monferno will go back to Paul, because well... Paul's not a bad guy, he's just a perfectionist. And he'll want him back~
^^" err, yeah, that could happen.