A couple of SUPER minor things I wish were implemented... phone customization options... wallpaper, ringtone, etc. And I wish the character held the phone up to their ear like in single player. It's little details like that that really help make the experience so complete.
I called a friend in-game who was playing Grand Theft Auto V and he was playing as Mike, and I was in GTA Online mode. It was really cool. Little things like that make this game SO fucking extra cool.