Hatsune Miku and Future Stars: Project Mirai lets you view augmented reality Vocaloid performances, but we don’t know much more about this feature at the moment, other than that you can take 3D photos of the AR models and include your Mii in the same picture as well.
Sega demonstrated the AR feature with screenshots on their Miku blog, although they didn’t show off Mii functionality:
The other feature they showed was one that was recently revealed in Famitsu magazine: the ability to write and share comments that are overlayed over videos, NicoNico-style, in the game’s Theatre mode. Here’s how that works:
Comments are shared with other Project Mirai players using StreetPass. Hatsune Miku and Future Stars: Project Mirai is slated for release on March 8th.