I'm not sure how much you're planning on leaving to modders or what your intention is with this game; but something like this could really take off if it's a good game in itself.
As amazing a full world like this is, it's something that will only be entertaining for a few hours at most if you're unable to interact with the environment. A flight sim\driving sim doesn't really make the cut...
Here's a few ideas you may want to consider that will make a huge difference;
1.
Interactive environment. It doesn't have to be completely realistic, if you run into a tree it's very cool if the tree actually breaks or gets knocked over (along with your vehicle getting knocked back or even taking damage), it's the difference between "Oh look, trees." and actual immersion into this world. They don't need to stay as knocked over tree-carcasses, just have a tree death animation and have it fade away, same thing with any buildings that may exist.
You may want to consider adding random pieces of terrain like boulders or rocks that can actually be kicked around, the more you remove the game from a static environment, the more it goes from a "cool" to "Damn! I want to show this to someone!" This is why cars on roads, birds, random animals, weather, even something (probably impossible) like avalanches are so sought after. We see so much potential in the game and would love to be immersed in a world, not just have an interesting environment to look at. We want to be part of it.
2.
The first thing I wondered when I saw the youtube videos was "Can I walk around?"
The vehicles are okay, but you really want to have a 'guy' that can enter and exit different vehicles and actually walk around on foot. Again, it's for the sake of immersion.
3.
Climates, and making it interesting to explore the world:
If every part of the world looks the same as every other part of the world then there's no point to actually exploring the world. Having some mountain areas, some desert areas, some flat areas and other slightly hilly areas with different types of trees and wildlife organized in different ways will greatly increase incentive to explore.
moreOften times unrealistic or very different atypical environments can be very interesting to explore while still being believable.
Don't just give us some static world we can only pass by, immerse us.
Wishful thinking: -Cities
-Clouds and weather
-Capability to have cities in the oceans and ocean life (coral reefs, kelp, whales) in a similar way trees are generated on the surface
-Capability to construct buildings and actually go inside of buildings
-Weapons and enemies that we can turn on\off
-Capability to go to other planets within the game
-Destructible buildings
-Caves
Extremely wishful thinking:-Towns that grow over time and become more advanced over time
-"Nations" with territories that expand and come in conflict with other territories, and they actually fight each other over it.
-Actual 'space physics', spaceships, spacebattles, SPAAAAACEE! :p
-The larger the nation, the more technologically advanced it is - going from wagons to cars, huts to skyscrapers, eventually a large enough nation can explore and colonize space. Maybe even have it ongoing on alien worlds that may want to come and forcefully colonize earth.
bombs,grenades, nukes that leave craters
I know it's just a small team working on this game, but there's so much potential with something like this that at the very least it's fun to come up with possibilities that could become part of it.