If Reeves did keep with the Nirvana influence, the track "Dumb" would be very apropos. Especially if they play up Bruce having to thrust himself into the world of socialites/dealing with the board members of Wayne Enterprises, and feeling disgusted by having to put on a facade.
"I'm not like them, but I can be pretend"
"The sun is gone but I have a light"
"Skin the sun, fall asleep
Wish away, soul is cheap
Lesson learned, wish me luck
Soothe the burn, wake me up"
That's just a few stanzas, but the entire song could be very fitting for the next film if they indeed go in that direction.