Trump is brilliant. Just like he allowed Ted Cruz to speak at the convention and let Trump supporters boo the heck out of him, he’s essentially allowing the same to be done to Romney. No sense having his fingerprints all over, when his supporters are in no mood to punish the man who did his utmost best to get Hillary Clinton elected.
I think at best Romney will get an ambassadorship, or tasked to fix the VA. Romney knows how to fix things, and the VA is a high priority thing to get fixed.