Roman Reigns is probably the one winning MITB. It'll likely be the catalyst for how he'll end up in the Brock vs Rollins match at Summerslam to make it a triple-threat.
I'd personally love to see Ambrose win it and then stalk Rollins much in the same way he did last year when he was intent on stopping him from cashing in the briefcase by any means necessary, only this time around it's Dean who has the briefcase and Seth who has the title, and Rollins knowing that Ambrose is always out there waiting in the shadows and ready to cash in at any moment will have him on edge and surely be more nerve-wracking on him than anything ever has been before.
Then again, the other part of me would probably prefer if Ambrose won his first world title in a much more distinguished manner, such as winning the Rumble and then the championship at Wrestlemania, or winning it at some other PPV after earning a shot at it through some other means (as opposed to a spontaneous cash-in.)