8. Bowser Jr.
I've included young Bowser due to my preference to him over his elder relation. He has also appeared in nearly all the Mario games since Super Mario Sunshine I believe and would make a welcome addition to the traditional ranks. He is also an obvious choice due to his paintbrush he often uses, meaning the developers could have a lot of fun giving him related moves and even include his Koopa Clown Car to add even more fun to the mayhem. I think it is very important in games these days to keep them fresh and light, rather then sticking with the safer but often more dull line-ups.