Honestly, I'd hope for a new Wii-exclusive IP, but it makes the most sense that it would be a MGS game because Snake in Brawl is an excellent advertisement for MGS, except that the only MGS game playable on Wii is a GC one.
I think they should do a TS Wiimake, ala RE4 and then port MGS4 to the Wii. Watch the hilarity ensue as it outsells the PS360 versions.