Excuse me, but all of the pairings you listed ARE trying to accomplish the same goal.
Mario and Bowser = Get Peach
Link and Ganondorf = Find Triforce
Samus and Ridley = Get Baby Metroid
Kirby and Dedede = Acquire all food, everywhere
Sonic and Eggman = Be in a lot of crappy games I guess
Megaman and Dr. Wily = Kill a whole lot of robots
See? Rivals.