I'm indifferent to Miis. If they're in, cool, if they're not, I won't lose any sleep over it. I think they're neat, I've really grown fond of them thanks to Nintendo Land and Pilotwings, but I don't consider them classic Nintendo icons and I don't think they need to be represented in Smash as any matter of urgency. I'd be fine with just having the Miis saved on the Wii U or 3DS appear in a Nintendo Land stage.
If they do get in, though, I don't think they should have customisable movesets. That would get complicated, fast. They would be better off with a moveset done in the same way as Mr. Game & Watch's - moves inspired by all the games they've famously appeared in (the Wii series, that sort of thing).