All faces in one MITB match, all heels in the other - I like the idea.
I could see Ambrose taking the case here. While it won't be immediate, this could be what really starts the breakup of The Shield. At first, it's just a good thing to know they have a chance to bring the WHC to The Shield. Then, Ambrose cashes it in and becomes World Heavyweight Champion. Eventually, Ambrose starts to behave as if the other two are just meant to act as body guards to him and protect him as world champion, and maybe this is after they lose the tag team titles and he scolds them for losing the tag team championships (treating them like they're just fodder, in other words.) And like a typical heel who recognizes the faults of others while refusing to see his own, he ignores the fact that he lost the United States Championship prior to cashing in the briefcase, although maybe there's a way to play that into the whole thing (Ambrose could have at least tried to help interfere when they lost the tag titles, whereas they DIDN'T come to his aid when he lost the US Title.... Or maybe they did, but they ended up costing him the title instead... accidentally, of course.)
This begins the dissension between them, as Ambrose begins to see it as being all about him rather than the three of them as a group. And Ambrose winning the briefcase at MITB could be the start of a long-term angle to all that.
Otherwise:
Swagger - No.
Caesro - No.
Fandango - Obviously a no.
Sandow - Not really.
Rhodes - Slightly plausible, but I doubt it.
Barrett - Maybe. My second pick. What other way to give him momentum after coming off one of the worst Intercontinental title reigns ever than to give him the briefcase to carry around?