I suspect it'll be Kevin White. The Raiders can claim they got a first round player back...even though it's Kevin White.
In a way that would help the Bears as they have a log-jam at WR and it frees up some cap space for Mack. Meanwhile, Gruden reportedly hasn't been happy with Martavis Bryant - and Jordy Nelson can't have many years left.
Other possibilities that spring to mind are Danny Trevathan or Bobby Massie as the Bears seem to have an heir apparent to both players.