Nobody should be getting any assurances about being the next HC in 2 years. Not BOB, not Mayo, not Steve, not anyone.
Let Mayo have his Asst HC title. If he excels at it and impresses BB and (more importantly) the Krafts, then they can make their decision when the time comes. If someone else offers him a HC job next offseason, then you have a decision to make. Not now.
Let BOB run the offense. If he does great, he'll get another crack at a HC job in the NFL with NE or elsewhere. No assurances need to be made beforehand.
I'm not a fan of thinking promises of this sort are being made so far in advance.