#1. Creative control
Out of all the things on this list, the option which has the highest chance of convincing Dean Ambrose to stay would be to provide him creative control of his character. It's very rare that any superstar nowadays gets creative control of their character and storyline, but it was the terrible creative direction on RAW that drove Ambrose away in the first place.
We can't really blame him, because being stuck in a creative rut can be frustrating for any WWE superstar. With that being said, it all comes down to how much Vince McMahon personally values Dean Ambrose.
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If he does as he should, it likely won't be an issue giving Dean Ambrose creative control. While it's unlikely that Ambrose would use his leverage to politick himself, what it can provide is a way to keep him satisfied, and an option for him to give a stern "no" to any storyline that he doesn't deem is right for him.