Thankfully this isn't true of the game, at least as it'll be released. But someone certainly had an interesting idea in regards to what non Mario characters they'd want to add:
Nintendo Power: If you could introduce another Nintendo character (not from the Mario universe) into Mario Tennis Open, who would it be? Why?
Takahashi: Characters from the Pokemon series, like Pikachu, might be a good fit. I think that might be a good way to get new players interested in the game.
Would you want Pokemon characters in Mario Tennis, presuming at some point Camelot were allowed to add them? Or would they seem horribly out of place?
http://gonintendo.com/?mode=viewstory&id=175658
Nintendo Power: If you could introduce another Nintendo character (not from the Mario universe) into Mario Tennis Open, who would it be? Why?
Takahashi: Characters from the Pokemon series, like Pikachu, might be a good fit. I think that might be a good way to get new players interested in the game.
Would you want Pokemon characters in Mario Tennis, presuming at some point Camelot were allowed to add them? Or would they seem horribly out of place?
http://gonintendo.com/?mode=viewstory&id=175658