#2. Promise him the WrestleMania 36 main event
Would it be absolutely crazy to promise someone a WrestleMania main event a year in advance? Absolutely. Is it out of the realm of possibility? Most definitely not. Charlotte Flair was reportedly promised the WrestleMania 35 main event.
Dean Ambrose's original plans for WrestleMania involved him challenging Roman Reigns for the Universal Championship. This would have been an epic match, though it likely wouldn't have main evented the show.
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Dean Ambrose is worthy of main eventing WrestleMania and if he's promised the spot, all WWE needs to do is make sure that there's a very consistent build towards the main event. An undefeated streak is one way to get him prepared for it, but WWE would also need to constantly keep him in high-profile feuds throughout his time towards the main event.
A high-profile win at the Royal Rumble and a guaranteed main event could certainly convince Ambrose to stay.