Yeah, Captain Toad makes more sense. I just used Princess Shroob as an example, because, although Bandana Dee is very recurring, arguably, the one game where his importance really stands out is RTDL. I guess I was a bit vague with what exactly I meant. No, there are technically no chances of whether a character will be in or not. (Of course Sakurai just puts the face of every Nintendo character on a roulette board and just hopes for the best!) But yeah, in reality it seems to be all about the compatibility of a character, chosen way before development. Perhaps I should rephrase what I said, to that Ridley seems to deserve the role of a newcomer the most, currently, imo. We got Bowser, Dedede, Donkey Kong, Ganondorf, who, are for the most part, the big baddies of their games. Ridley is to me, the only deserving Metroid rep, and has been wanted in every game. Although I do like Bandana Dee, he's a fan favorite, I suppose. But, imo, Ridley should be priority over Bandana Dee, as he only began to rise in importance over the recent years.
Also, thank you! I'm having a bit of trouble adjusting here, but I signed up cause /r/smashbros.