At first I didn't think it was very likely that you would be able to carry over hats from StreetPass Mii Plaza, but after looking into it you may be on to something after all. That top hat in the 3DS screenshot? It looks identical to the one in SPMP, right down to the pink/purple band. Then I noticed that the football helmet is indeed identical to the one in SPMP, as well! And although I can't find any photos of them, I do see a straw hat and a cowboy hat being listed as one of the hats you can get in that game. On the "against" side of my findings, there is no hibiscus flower available in StreetPass Mii Plaza, only a sunflower. Also, there is no alternate cowboy hat as seen on the female Mii next to the one in the traditional cowboy hat. Most trivially, I'm not sure why the top hat in the Wii U screenshots is purple as opposed to black (nor am I a fan of the change at all). Still, it would certainly seem that, if nothing else, some designs were directly taken from SPMP. Now I have a whole new personal concern: will there be a simple crown available? As in a gold ring with spikes, with no fabric in the middle? I'm really hoping so, I have some ideas that would work much better with that type of crown than with a regal crown. Fingers crossed.
Oh, and this isn't really backed up by much compared to my other thoughts, but given that we've seen a character with a long coat, I'm pretty certain we'll see costumes with capes. And if we get cowboy, sports, and sci-fi outfits, I am willing to bet a nice little chunk of money that we'll see a martial arts outfit.