Just the part about an interesting backstory and original moveset.
I know exactly where you're coming from, and while it's not my personal opinion on Mii's, I do feel it about other characters in Smash. People adapt though. I hated the idea of ROB in Brawl, and playing as him made me appreciate him in a different way.
You are right to say that making the Mii's interesting may be tough. Their moveset would have to be amazing for them to be worthwhile, but seeing as they've been in a ridiculous amount of games now, I think Sakurai could be really creative.
Perhaps it's as suggested, that the Mii's wont appear because they are being represented in stages... but in four Mii stages, we've seen no Mii's. It's like Pyrosphere time four.
Rosalina has a backstory, two big games she's fairly central in, and she's practically a goddess of the cosmos, so it's perfectly feasible for Sakurai to invent a move-set based on
space for her - if that's not interesting, I don't know what is :| I think she's very, very far from being as dull and throwaway as Miis. I can understand people being disappointed she got in over other Mario characters, but that's a whole different argument.
Yeah, I suppose it's still possible Miis are merely being represented through stages, like Animal Crossing last time except more-so, and I'd really love to believe that... but I also have to admit that the Leaky 3 leak is starting to look more and more legit, which is really what bums me out here. Ah well, I guess if I start expecting them now I'll be less disappointed when they actually show up XD
I was pretty disappointed by ROB at first, too, and I still don't really like playing as him, but at least he is a very unique and interesting piece of Nintendo's history that brings something somewhat different to the table - I can accept his inclusion based on those merits even if I'm not crazy about him otherwise.