I could go the easy route and say "It's the U.S. Title. It doesn't need a cohesive storyline.", but that's not my way.
Have Bo come out (as the "blue chipper") and have a few random matches where he goes over a few guys (Ryder, Sin Cara, maybe even a slightly higher profile guy like Curtis Axel or Kofi Kingston), then have him talk cut a promo about how Ambrose hasn't defended the U.S. Championship in a while and that it's not right that he gets to carry that prestigious title (you'd have to have him really put the belt over here, but that's just smart wrestling psychology anyway) and how at Extreme Rules (or whatever PPV's a week away), he wants to challenge Ambrose to a match for the U.S. Championship. Then I'd do something really amazing: I'd have him pin Ambrose clean after they'd had a good 10 - 12 minutes for a match (it wasn't unusual to see 10 - 12 minute matches for the European title or WCW Television title a few years back, so it shouldn't be a problem here). Have him successfully defend the title (again cleanly) against Ambrose the next Main Event or Smackdown and set him up with a longer feud against somebody who's ostensibly a face and go from there. Let him carry the belt and actually make it a plot point that he defends it regularly on Main Event and Smackdown (a defense every 3 weeks or so wouldn't be amiss, plus him participating in random tag matches or non-title matches where he continues to push how great he is and how his pedigree is so good (it legitimately is....so that's almost shoot). But he starts having to cheat a little to win (holding the tights, putting his feet on the ropes during a pin, that sort of thing). The fans'll turn on him and start hating him (or he's the next Stone Cold Steve Austin....I guess anything's possible).
Meanwhile, Ambrose starts feeling like the weak link since he got beat by the rookie for his title and (unlike it becoming a comedy angle like it did with D-Bry) his "lunatic fringe" side starts becoming bigger. As Reigns moves on to more singles matches and into the upper midcard in that way, put Ambrose as Rollins' tag partner, with Ambrose costing them matches left and right. Let Rollins show his frustration, but don't have him do anything destructive to Dean. Then, one night, Dean costs them a match and Rollins goes to "lift his spirits/tell him everything's okay" and Ambrose snaps and attacks Rollins, leaves him lying in the ring (obviously Reigns isn't out there for this match). Have Rollins confront Ambrose one-on-one (he would tell Reigns to stay in the back and this was between the two of them) and Ambrose attacks him again, this time leading to a fight, which leads to a feud between Ambrose and Rollins. There's your final dissolution of the Shield. It also works since the Shield has become looked upon as faces by the fans.
So, you've brought up a guy from NXT to the main roster and gotten him over (and rehabilitated at least some of the credibility of the U.S. Title), split the Shield into three separate parts by separating Reigns from the others and setting up a feud between Rollins and Ambrose, so you've basically pushed four guys to a higher level than the one(s) they were already on.
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