Okay, since we are merely guessing, I might as well give my two cents worth.
I believe Braine has already filled Gailey in on the strengths and weaknesses of each assistant. He also knows which assistants are most and least respected by the players. Since I don't know all the assistants, I will give an opinion on each I have seen on the boards.
I doubt Mac will be retained. It is ironic, because the players zealous efforts to retain Mac, may cost him his job. It could cause friction between the players and the new coach if Mac is retained. The players would have a tendency to go to Mac instead of the new coach regarding problems. This never works.
O'Brien might be retained as he is considered a future Head Coach and has learned a great deal under Friegden. Law-bee may be right, he could be named the offensive coordinator and relay information to Gailey from the press box where he would be viewing the defensive formations. It would be another good learning experience for O'Brien under Gailey.
Roof might be retained if he is willing to take the linebacker coaching position or something similar. I don't think he will be retained as the defensive coordinator.
I feel Gailey will ask Kelly to stay, but not at a coordinators position, so he probably goes to Vanderbilt.
I really have not read much regarding the remaining assistants, so I have no opinion on them.