That is a big problem. Fortunately with having Lesnar booked as a monster, you CAN afford to throw midcarders at him for a few months in between the Big 4 ppvs. Dean's someone who could give him a good three month feud, because Dean being "crazy" and using weapons is a great equalizer. But you're right, there's not many other options.
Of course problem is with Ambrose is that while it's a fresh matchup, Ambrose isn't a heel. That's not compelling. Fuck.
Authority Cena and Rollins is the only good option in that scenario.