I don't think Barrett is going to create a new action for a limited run. Short or long action standard rounds. Anything larger or smaller probably isn't in the cards. Don't think a left hand action is coming out for a limited run either.
They could keep their short action and add a mag face bolt for the SAUM... Actually would work quite well as their mag box is longer than a standard Rem 700 SA....
Honestly, I can't think of a better fit for making the most out of the 3" mag box.
7-08 with heavies should be good to go as is.
6.5 Creed won't have issues. Could go .260 but why bother?
6.5 SAUM would be really sporty in the recoil dept however, and I'd not want one.
