Man, lots of Schadenfreude going on these last ten pages or so. >_>
Anyway, I have a crackpot theory.
What if the Streetpass and Tomodachi stages are one and the same? Which is to say, there is one 3DS Mii-themed stage in which you visit multiple Mii-centered locales (Streetpass to Tomodachi to Mii Channel to etc.)?
It could very well be two distinct stages, and I have no issue with the concept. On the other hand, I also wonder if this might be misdirection, given how little information we got. In my crackpot scenario, it would be a Castle Siege-type soft-transition stage of sorts.
The Newcomers thus far appear to have one stage per system (Smashville + Tortimer, Wii Fit Gym + ???, Punch-Out Wii + Punch-Out 3DS)***. Would Mii get more, on account of being the ostensible mascot of the past two generations of Nintendo consoles (thereby making them deserving of more stages than other Newcomers)? Right now, we have Wuhu Island on the Wii U, so the only way to settle this crackpot idea would be to see a second Mii-centered Wii U stage (e.g. Nintendoland?).
Just something that came to mind. There's probably no merit or basis to it, but still, a Mii-flavoured smorgasbord stage could be interesting.
***I'm referring to Newcomers whose series have not yet been represented, hence why I excluded Rosalina. If you insist that she have a 3DS stage, then Rainbow Road is as good a one as any. And I expect Mega Man to have one stage on the 3DS, whether it be 3DS Wily's Castle or something else.