Maybe the next Star Trek movie could take place in the 'Mirror Mirror' universe. 
     This is a way they could bring Shatner back.
     He would be the captain of the 'Mirror Mirror Enterprise'.
     He could have a goatee.

     Since they reversed Kirk with Spock yelling out "KHAAAAAAAN!",  they could certainly do some more role reversal.
     Shatner should probably lose some weight.  He would look more like himself.

     There is no reason why they shouldn't bring Shatner back.  Maybe he could give some tips to the actor playing the new Kirk, who more or less looks like a young Kirk, but doesn't have Shatner's acting chops.