Hm, tough question
All options seem nice. But if i could propose one... I see people asking for tutorials where making a track is fairly easy. I would say nobody would need more than a day to make a track like Valencia (only layout, no elevations). The hard part is making it a "track" One you would want to drive ingame because it looks populated. This takes far more work than creating the track itself.
So i real cool feature would be the ability to insert basic objects into your track. (model + texture) I'm thinking of bridges, pitbuildings, crowdstands. You name it. There should be a basic format for those objects. So you could export existing buildings and objects and easily include the in "the gp4builder library" and other people could make basic buildings and objects you could include in new versions.
This way, way created a track with mere clicks (because thanks to all your hard work on features that's what it has become) you can just as easily create a living track (add a few bridges, set up the pit building, create a crowd and it looks alive
)
Just an idea that popped into my head. :P