This shows we may have some new characters! There's Diddy Kong, Plessie, E. Gadd, Birdo, and Kamek. I, for one, am very happy for these choices, as I wanted to see Kamek in Mario Kart for a while. I'm not sure about the other newcomers though, but they work.
This shows that Shine Thief and Bob-omb Blast from Double Dash!! may return as well. Shine Thief was really fun, and it seems that Nintendo is listening to their fans when they said they hated the new battle mode. Perhaps we may get a pack with original battle stages?
Remember that this is just speculation, as it has not been confirmed yet. We will probably see it announced at E3 tomorrow, but let's just wait and see.
EDIT: The battle mode one was confirmed to be a fake :( But let's hope Nintendo shows something tomorrow.
EDIT again: Well, there's something else: