I'd like to see him get a PPV match with Seth Rollins. In fact I think one of the members of the Shield winning the belt from Cena would be good. Cena could beat one of the Shield members at one PPV, but then lose the title to another member at one of the next PPVs. The only problem would be that it would make one member look like a leader and the others look less strong, but there's ways around this if they continue to book all three members well enough in other matches.
And I know this is going a bit off the actual topic, but the Shield having control of the WWE title and WWE would make for a nice storyline and would be a nice way to have Punk turn face - he could claim they are stealing his shtick from 2011.