According to someone who claims they saw a new Grand Theft Auto V presentation behind closed doors at a GameStop managers' conference, Rockstar Games is working on some fairly significant improvements to the game. Here is what NeoGAF user, The Shogun, says he saw during the presentation.
1. I saw this at the GameStop managers conference in Anehiem. Rockstar gave a 45 minute closed doors presentation.

2. They showed us multiple locals. The city, underwater areas, country side etc.

3. Foliage has been completely reworked. Tree barks been redone, individual grass blades, wild flowers etc. This applies to underwater areas as well. Completely reworked coral reefs and underwater plants.

4. Draw distance now is INSANE. One example they showed us was the main city at night. They showed it on PS3 first amd showed us how all lighting at a distance was pre baked. In the remaster its all pre loaded in real time and is dynamic. You can see lights and traffic flow from massive distances.

5. Water rendering system has been completely redone. Rivers / Ocean / rain are gorgeous

6. No release date given. They simply said soon.

7. Animal texture work has been 100 % re done.

8. Anti Aliasing looked great with little to no jaggies. Looked 1080p to me. It was demoed on PS4.

9. Promised "new" content. One example was new songs / soundtrack / radio stations.

10. Best remaster Ive seen by far. I literally gasped at the differences.

If this is true, those are some very significant changes being made there for the upcoming PC and current-gen (PS4, XB1) release of the game. It's not just a simple up-ressing of existing assets, they're actually adding in more content to the game.