There has been at least 3 threads about future DLC ideas over the last week.
North Yankton DLC
Won't happen because the map isn't big enough. I've loaded it up using a PC trainer and it's seriously not that big. Highly doubt they will expand on the map, since they've already said they won't be working on more heists. And developing a map is more difficult than working on heists. Maybe they could create a regular contact mission there so we can relive the place.
New radio stations
We don't need any more of these. Better if the devs worked on something else. They need to buy music licenses, and I'd much rather they spend their development expenses elsewhere.
Cars
I'm hoping for more mod options, and interior customisation for all existing cars in the game.
Contact missions
What I'd really like them to do is redevelop the stealth system and make it so we can stealth missions for a bonus reward. It'll give each mission more re-playability as there are more ways you can choose to handle the situation, rather than just going in heavy and loud. Also achievements for every mission with a one-time cash bonus, such as "paint a car pink before delivering to Simeon".
Clothing
More colour choices and textures for existing clothing models, make crew colour an option for all clothing textures. Better compatibility with different items of clothing. I want to wear a scarf without also wearing an Ill-Gotten-Gains jacket. Improve the existing clothing models. Make the Ill-Gotten-Gains T-Shirts longer for females. Add a proper dress or mini-dress for females which looks good.
Matchmaking
Needs to be improved for PvP game modes. It's currently difficult to find a random game mode with more than 4 players.
Heists
Hopefully they do change their mind and develop more heists. It'll be tough playing the same 3 (Fleeca and Prison Break don't count cause they pay terribly) over and over again to earn GTA$.
Character appearance changes
A plastic surgery option is one which many players would want. Improve the sliders when it comes to creating your character's face. Add more sliders and make the sliders more extreme. Also raise the breasts on the female character model higher (she seriously looks like she's not wearing a bra). Maybe add the option to change body proportions on your character model so she doesn't look like a skinny crack addict. Better make-up options and more controller sliders would really help too! New longer hairstyles for females as well. Maybe different variations of the ponytail, so every female character online doesn't look the same.
Mechanic
Please make it so you don't have to walk three blocks to request a new personal vehicle. Also add the option to repair/wash your vehicle on the spot from the interaction menu. I know it's not realistic but it really helps! Add the option to immediately retrieve your personal vehicle through insurance after it gets impounded by the police. Currently you have to wait for it to get destroyed before you can call insurance for it.
Weather and enviroment
I know they left out a lot of the street detail from single player when online was first released on old gen consoles. With new gen and PC, there is no reason why they can't add more diverse weather conditions (like lightning, fog or darker nights) to online and a bit of the extra street detail.
UI and HUD redesign (especially for PC)
Add more mini-map zoom options. Allow us to lock a zoom level on PC. I know you can do this on console, but not PC. Add more keyboard control bindings for PC. Much of the stuff in the interaction menu can be bound to a key meaning easier access. Redesign the interaction menu for both console and PC, re-order the options to make it more user friendly.
Most of these suggestions are improvements to the game rather than actual new DLC content, but I'd be really happy if they could add these changes to the game.