I'm going to focus primarily on the online but my suggestions would be:
(Not in order)

-Heists, available every 2-5 hours and you can earn between $100,000-$2,500,000
-The option to customize atv's and vans
-The option to add insurance to any road legal vehichle
-Casinos
-Drag Racing
-The ability to gift cars
-Pink Slips
-Wager Races
-Boat and Plane customization
-Personal Hangars and Docks
-To be able to change your characters features, e.g skin colour, gender and age
-Skate Boarding
-Bicycle customization
-House Customization
-More options of house, e.g bungalows, actual houses
-Ability to own multiple garages
-Ability to own a garage and house at the same time
-Add better paying jobs and a more variety of jobs from lester, martin, simeon etc, including racing jobs or towing etc
-Music Player on the phone to listen to music while on foot
-Music dlc to add more recent music to the radio stations

Well thats all i could think of, if you have any more suggestions then leave them below. I will edit this list as i think of more. Also let me know what you think of the ideas so far