I really hope they keep the custom soundtrack feature. One of my favorite parts of GTA on Xbox was uploading your own music.
As far as radio stations go, I wanna see a good rap station(This is a must. It's in NYC. No 50 cent garbage though, I wanna hear Nas, Biggie, Rakim, that real stuff.) A good funk station, and a good soul/R&B station like the one in San Andreas, with some good James Brown or D'angelo, maybe some Jill Scott.
I have faith in R* to make good decisions, I generally like the songs they pick on the radio.