Nintendo president-face Satoru Iwata has stated that the 3DS Mii Plaza will be updated with fresh features by the end of the year. By that, I hope he doesn't mean, "There will be more Mario hats."
The Mii Plaza is currently home to some good, but ultimately half-finished, ideas. You can play two little games with the Mii characters, but they're extremely limited in scope, barely interactive, and generally pointless. Also, once you've played with all the Miis you've collected via Streetpass a single time, they cannot be used again -- unless you get a second Streetpass download.
With real videogames being in short supply for the 3DS, the in-built software could have really gotten some life out of it. Hopefully Nintendo puts in some features will get some independent longevity going.
In addition to the Plaza update, Iwata said that more games are coming that will make use of Miis. You're all excited about that.