The best character, Giant Claw!
Seriously, probably Captain Toad, Fawful, and Olimar because I'm insane and actually like the non-Mario characters.
Also, I don't see how putting characters in a dropdown would change anything. They'd still be there.
The thing with the koopalings is that there are seven of them, and they're usually pretty samey. They could make some of the koopalings more distinct, but that would be strange, with some recieving more limelight than others.
I believe that Mario & Luigi deliberately uses different shades (or possibly tones, this isn't really my area of expertise) of red and green to avet that very problem.
Also, RE: Super Guide, shining stars are a good point, that can unfairly impact completionism. Also, I think the super leaf...