Two things...one of which is a virtual lock: Angela, Lupita, Forrest and them all coming back.
I imagine he'd want to be a part of that crew.
The second thing....? Well, remember that "I'm King Kong" Speech in Training Day?
A script that lets Denzel go off on Best Actor bait speechifying for at least half the movie.
Basically BP2 being "Fences" with fight scenes and SFX.