I personally think Nintendo needs to create its own suave, super-spy first-person shooter akin to Goldeneye 007 (without being, ya know, James Bond). Make it have ALL of the multiplayer options that the best online Halo multiplayer game has and then some, and give it that Nintendo quality feel. Aaaaaand then, market the living holy jeebus out of it, proclaiming it to be the end-all/be-all Halo killer.
Then Nintendo has its own serious first-person shooter.