I've had the same thought, but his lateral movement defensively is way too slow against quicker SF's. I would use him some at SF against slower SF's, however. It would depend on matchups.
A good 3-man rotation at PF and C would give Millsap plenty of PT, assuming Favors can also play C, which I think he could down the road. I do see Millsap as a 'super-sub' 6th man more than a starter. Same with Miles at SF.
Without Deron here commanding huge $$, I see us re-signing AK, and he would start at SF.