This really depends on whether the intent is to prove that Nintendo can hire a Western studio to develop a Nintendo published FPS using one of Nintendo's many IPs, or to that Nintendo's 1st party studios in Japan can show everyone how FPS are done or at least do something new with the genre.
If it's the first, there is definitely an untapped market for FPS games on the Wii, albeit smaller than that of PC, PS3 or Xbox. But the FPS genre just won't ever be one of Nintendo's bread and butter platinum sellers. And farming an FPS project out to a foreign studio with substantial experience in FPS games wouldn't really prove much of anything other than the limits of the FPS market potential on the Wii.
If it's the second, I'm not really convinced that even if they could "show everyone how FPS is done" in a market where companies know the genre so well they have built their success upon nothing but FPS games. At best, they could come up with something different that steps outside the norm for the genre, like Little Big Planet style "paintinator" game play, or even an "FPS" game like Portal.
For one thing, Nintendo is very much a Japanese company that makes games that reflect its roots and FPS games have always been one of the genres that simply never caught on in Japan as they did in other global markets. One could even say that the Japanese just don't "get" the appeal of FPS games.
Any attempt at such a game would really just be an exercise in showing how they can adapt a genre they have almost no experience with into a game with the same broad appeal as say Mario Kart with racing games.
