As seen in the trailer released today, Mario is seen playing tennis against a Luma from both Super Mario Galaxy games. To further add to the excitement, the stage is in fact a Galaxy themed court, complete with Star Bits, a planet, and even a Warp Star.
The question remains whether Luma is indeed a playable character. The scene previously depicted was a Special Game, and the court and character may be unique to this mode. However, it is hard to believe that the developers would create a character and court that are both fully functional, and leave them in single mode. Chances are they will both be available, or at least unlockable, in the main modes.
And to further the discussion, what does this mean for Rosalina? Fans have been clamoring for a game outside the Mario Kart games where Rosalina makes a playable appearance. Will Mario Tennis Open finally be that game?