I don't see any heel turn happening.. I believe Ambrose will turn heel but not now, it will happen before the year is over.
For the Payback match here's a simple way to book the match - Ambrose is out first but a returning Big Show brutalizes him during his entrance. Reigns comes out and gets rid of the Big Show. Dean is out for the match while being checked on by the doctors.
The remaining three continue the match in the ring and eventually Ambrose runs back out and they hit the triple power bomb on Orton to get a huge pop. Reigns goes for the spear on Rollins but hits Ambrose and Rollins capitalizes on it somehow and pins Ambrose.
This sets up a potential feud between Dean/Seth, Dean/Reigns, Dean/Big Show.
While I like the idea of Big Show getting involved just to create some heat, having Ambrose out of the match before it even starts is tantamount to creating another Royal Rumble boofest.
Fans are overjoyed that Ambrose is finally getting some credit. Do I think he'll win? No, he won't but he has to be see to be given a fighting chance. Out of the four of them he is the most over, and no one really wants to see Reigns/Rollins, or Reigns/Orton or Orton/Rollins, they want Ambrose in there screwing everything up and throwing a wrench into the works.
Like I said before, Ambrose is crazy over despite winning hardly any matches. For some reason the crowd loves him, might be the unpredictability factor he brings to the ring, whatever it is it works. He has a natural charisma and has connected with the crowd without apparently even trying.
Putting him into a feud with Reigns would be the worst thing they could do to Reigns. All the work they've put into getting him over, would be gone in a flash. Heel or face, Ambrose would be cheered so keeping them apart is the best option going forward.
Dean Ambrose is one of the best things going in the WWE right now. While others have their characters set in stone, Ambrose is the one who will break the mold every time, and surprise us all. So it would be a huge misfortune for them to just use him as a cheap ploy for any of the other three.