I'm pretty much with Hamler on this. Dean Ambrose had a lot of things going for him but he's sort of drifted more into the realm of an afterthought than the other Shield members. Rollins is a workhorse, someone that's going to be counted on to deliver strong matches night after night for the next decade, Reigns is Vince's pick and, like it or not, that's how it's gonna be and Dean is someone who's sort of fallen by the wayside. Of course, some have pointed out that some of the fault may lie with Ambrose himself as more than one star, writer, fan, insider, etc. has accused him of becoming lazy and complacent.
I'd personally like to see Ambrose return to the heel he was during the first year or so of the Shield. He was colder, more calculating and more opportunistic in a way that I thought suited him. That's not to say that Ambrose hasn't been entertaining, he has in my opinion, but I also don't think he's really lived up to his potential; I think he's gone as far as he can as the "Lunatic Fringe" as a babyface and I'd like it to be him who ultimately undoes the Shield the 2nd time around. I'd settle for Reigns if he were going full out heel but that's not gonna happen anytime soon, if ever.
As to how it goes down, both the sudden meltdown and slow burn have their advantages. When Rollins suddenly turned on the Shield the night after they beat Evolution at Extreme Rules, nobody saw it coming and it immediately helped establish Rollins as a top heel with great heat. If they went such a route, then I most certainly wouldn't want Ambrose aligning with a heel authority figure, don't ally with Stephanie or Triple H as it'd be nothing more than a rehash of Rollins joining the Authority. Keep the loose cannon aspects of him but do away with the goofy mannerisms and up the ruthlessness. A slow burn can also be fun if it's executed in the right manner; if they remain a group to WrestleMania, or past, then we could see Reigns as Universal Champion, maybe Rollins could be the IC champ after winning it in tournament, ladder match, elimination match, etc. and that would leave Ambrose sort of in limbo. Both of his brothers had gained singles success and, barring separation via a Superstar Shakeup, they'd have the only singles titles, Ambrose is too big to be a Cruiserweight and it'd be seen as a major demotion for him I think, so eventually snapping as jealousy consumes him is a possibility.