Yeah, adding a new member to The Shield wouldn't make sense. Reigns is already being groomed as the next big main eventer and once he's gone, that leaves only Ambrose. So unless they wanted to keep the group around with Ambrose and two new recruits (tuuurrible idea), then The Shield is pretty much done after Reigns and Ambrose split. It's had it's life and to be honest, I'm eager to see all three guys will look like when they're not all just wearing riot gear 24/7.
Anyway, being that Reigns is the guy designated to be the biggest breakout star here, it's pretty obvious that he's gonna be the one left standing to fight the whole battle by himself in the end. Which means expect Ambrose to turn on Reigns when it comes time for Reigns versus Hunter at Summerslam. Before that time arrives, though, this is kinda how I expect it to play out - Hunter tells Reigns and Ambrose next week on Raw that their buddy Rollins saw the light and realized that there's much more of a future in being on good terms with the boss than in fighting against him, and he tells them that the door is always open for them to follow suit and do the same. They rebel though, and the result is that Ambrose fights Rollins in a singles match at the PPV and Reigns competes against Orton in the MITB match (assuming Reigns isn't put on the sidelines because of his possible real-life injury.) Perhaps a couple of rematches at Battleground as well (Reigns/Orton, Ambrose/Rollins.) Then it's to Summerslam, which I mentioned above.
Fast forward to Hell In A Cell where Reigns defeats Hunter inside the cell (no idea what goes down at NOC.) Then it's off to Survivor Series where Reigns and four partners defeat HHH/Orton/Ambrose/Rollins and a fifth partner (a one-time thing, similar to the Wyatts teaming with The Real Americans last year, or Snitsky and Edge teaming with Evolution back in 2004) in a traditional SS elimination match. And that should bring this whole rivalry to a culmination point. I'd also expect that Rollins will defeat Sheamus for the United States Championship somewhere down the line, and Ambrose could end up winning the IC Championship from someone, too, if Barrett drops it to another babyface before that time.