Heel turn (RAW)
We had all expected to see this happen when the Shield reunited with Ambrose doing what Rollins did during their first run. That plan had to be scrapped due to his injury but WWE can pick it up from where they left off. Ambrose could turn on them when they need him the most setting up for an explosive feud with his Shield brethren. He is a lot better as a heel like Randy Orton and as such this is the safest way to go for WWE.
As a tag partner for Seth Rollins (RAW)
If, however, Reigns does come out of Summerslam as the champion then a Shield reunion would be off the paper and instead Ambrose could team up with Rollins and help him in his current feud against Dolph Ziggler and Drew McIntyre. The current storyline and teasers on RAW points to this being the most likely scenario to happen. This could also set up a great scenario for Ambrose’s heel turn where he sort of repays Rollin’s back for his betrayal during the original Shield run.