I'd honestly rather just get an indie game area. Well, "indie game," since that term tends to be rather vague; the really lesser-known games that just get lost in the sea of all the other titles on the site.
Assuming that's not something WM/GB is considering, definitely Mass Effect.I played through the first game at least five times. FIVE TIMES! I even bought the bad add-ons for it because I always want to see more of it.
If not Mass Effect, then perhaps consider some other games that will be coming out soon enough such as the BioShock franchise (despite the second one being lackluster, it might work). Another route would be going for an insanely popular franchise that has countless wikis already such as Zelda, Mario, Halo, etc, but I don't really see the point in that instance. Yes, GB are dedicated people, but the truly insane and intricate Zelda/Mario/Halo people are likely to stick with what they've been originally contributing to in the first place (not to say there aren't any on GB, but the regular users definitely outnumber these people).
What I think the community could get behind is what got them into the wiki submissions in the first place: humor. Hamburgers, Princess Debut, whatever gets the laughs, the submissions will definitely come. Cooking Mama is what comes to mind, but I'm certain there are other franchises that'd be just as good and (hopefully) better in this respect.
Hope this helps.