Looking good. But I got an idea for this to make it unique. You should consider adding the beta multiplayer maps to the game. They were left over on the iOS version of GTA III and made by Rockstar.. Now, I would put them where they won't affect the normal map.
A few places:
1. North of the mountains in the upper part of the map. That is if you don't plan to add anything back there as we can fly back there now and see the unfinished map.
2. Way way way way up in the sky. High enough so you can't see them from the ground.
3. Some of the maps are indoor maps. So you might be able to incorporate the indoor maps and put them inside of buildings on the normal map, and some may work under the map as well. However the outdoor maps you may need to pick 1 or 2.
Here are some screens of them that Frank.s posted in the VC 10A Modding topic.
If you make MP for this, you could have the areas be each of the MP maps (Except 1 as its missing the IMG), Portland, Staunton, Shore Side Vale, and LC as a whole.