I would keep it simple... community can make levels in Unity's own editor... then all problems are gone... I think that it will be a lot easier to make levels in Unity as I can do modelling in whatever software I want and load it to Unity's editor and then save them as Unity's and simply load them in FPV Freerider... The editor is there for us all... and it's free... no need to complicate.
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That is something weird as your levels wasn't maked in editor, you limit possibilities of your product... just add possible to load external models with textures and coordinates to spawn.
I'm very happy that you maked possibility to load external maps too, thank you! I have knowledge in 3D modelling and I have cool real places near me to fly so in couple weeks new maps will come! You're man!
I really like FPV Freerider Recharged... just like in real life... I would change locations to fly. Please consider making extremely huge map like city with parks etc... or some sort of simple places generator based on predetermined distances between trees and other objects (to match for flying) with couple types of terrain... or option to load external maps - to allow grow community generated content.