Obviously new vehicles, clothes, missions, maybe a new character or two, with their own special ability and missions. Also, an automatic respawn option for the cars you've parked on your own parking zone. I always lose the cars I've saved. And a colour palette for the Pay'N'Spray would be nice. Skipping 30 hours to see the 10 colours that you did not want is a bit unrealistic and inconvenient... That is what R* were trying to avoid, right?
And the split-screen freeroam thing would be great too.
Although those the auto-respawn and the colour palette shouldn't be DLC IMO. More like an update or something.