You can never rule anything out with Bill. Hightower is a middle LB thumper who rushes on 3rd down. If Bill think's he can get some of the early down production out of Bentley and the 3rd down rush from a combination of Rivers/Langi/Van Noy etc then he may pull the trigger. Hightower's health plays into this too. Perhaps they know his long term prognosis isn't great.
Now I don't think they just trade him for nothing but if someone offered a 2nd rounder or better then Bill may look at it.
What is interesting is they have a lot of similar style LBs - Hightower, Bentley, Van Noy, even Roberts. They could do with finding a 3rd down cover guy and having less of the early down guys.