For a while now Junior's been one of my favorite video game characters. If he's in I'm definately going to attempt to main him. I'd say his chances are pretty good (to say the least); he's the perfect newcomer to represent the Mario franchise. For one, there's very little good choices for Mario newcomers save Junior and Geno, but aside from that lack of choices he does have a whole lot going for him as well (and I could address that, though I think Gimpy did a pretty good job of that in the opening post).
Anyways, there's a somewhat prominent argument that's frequently used against Junior that really bugs me though, and it's that Junior is a potential Bowser clone, or that his inclusion would mean less variety amongst Mario characters. I've seen it in this thread as well. I mean, I can somewhat understand the reasoning, but the reasoning itself is elementary if you ask me. Junior is (obviously) Bowser's son, naturally he's going to look like Bowser. Does that take away from his move-set potential? Not in the least. Neither would I say that Junior's inclusion lessens variety of Mario characters. While he may look like Bowser, his character has been explored in a good amount of depth in Sunshine and even in Mario Superstar Baseball and Galaxy, and he's an entirely different character than Bowser. On top of that, he's important to the franchise itself, serving as the main antagonist of both Sunshine and the New Super Mario Bros, though also making several other appearances in the Mario sports titles, Galaxy, etc. Personally, I'd say Junior's potential inclusion alongside his father would only be fitting.
