If your squat is more of an olympic squat than it is a "powerlifting" squat, you may consider replacing the BBB back with front squats; really hammers the quads, and does a good job at strengthening the entire back which will help you stay upright in the back squat and can also aid the deadlift in the same respect.
Good mornings are a good choice but if your looking to do something else you also have RDL's, sumo deads, conventional deads, paused deads, etc. Though if you choose conventional deads, make sure you start light!
When I did 5/3/1, I personally liked doing BBB squats on deadlift day as squatting just once a week left me feeling like I almost needed to relearn the lift every week. Good mornings after squat.
Just a few simple ideas. The assistance work is important, but not to the extent that some think it is. Unless you push assistance too hard, you really can't go wrong with any of the big lift's variations.