The only things I see that need to be updated in the engine is the Water and Distant Ground Textures.
Well I mean a graphics overhaul will be nice but these are at least reasonable. Better LOD gfx for distant textures cause as of now they are kinda just blur's. As for the water, no matter the altitude it looks like I'm standing over a lake. The water should look more distant and not move very much at higher altitudes. Just my opinion.
EDIT. Weather texture update would be nice too. I.e. clouds rain snow ect.