I have to agree with Ian Sane, I don't think Camelot can get the game out before the Cube is finished.  For that matter, has anyone seen any recent mention of this game?  I don't remember Camelot discussing it since Mario Tennis and Golf were announced, but if someone has a link that can prove me wrong, please share.
As for Mario Tennis and Mario Golf, I think Nintendo is going with it's strengths.  The Mario games are huge in Japan (the GameCube came within 1000 units of the PS2 during one week shortly after Mario Party 4 was released) and even if Camelot made a great RPG for the GameCube, it would still be competing against dozens of RPGs on the PS2.  Still, I have to agree that I'd like a new RPG more than Mario Golf.