As mentioned above, the squat, Dl, cleans, Hi-pulls, BB rows can only be improved with lower back work.. Some gain an extra 10b to 15 lbs on their squats fairly fast. Also improvement in overhead pressing.
Also want to work the abs with serious intent, meaning using heavier weight and a lower rep patters. Abs and the lower back support each other, and working both equally hard insure equal muscle balance, encouraging more strength.....not weak link. Probably could throw that lifting belt away, the one that some guys get addicted too (and tend to wear incorrectly anyway).
Having a bar on the shoulders (GoodMorning for example) and holding the bar in front of the body (SLDL's for example) give different dynamic to each movement. Extensions are excellent for most BB'ers. An exercise that would do well to become a habit in training sessions.
For unusual heavy development and back thickness, checkout some of the Olympic & power lifters.
Good Luck.