as raspberry pi is getting more and more power, I thought of a version for RetroPie integration?! Or just like LibreElec, small Linux environment just to run FPVfreerider? 

I agree it would be a good idea, however -

Unity (that is used for development of FPV Freerider) can only compile executables for x86 CPU linux distributions. As far as I know all raspberry pi computers are using ARM processors and therefore cannot run x86 executables.

Unity supports UWP export and thus can target Windows 10 IoT on the Pi. Assuming most normal people are using Raspbian or similar, that'd basically mean a dedicated memory card just for FF, so while technically possible I feel it'd be pretty inconvenient. Not to mention unlikely to be worth the author's time to set all of that up just to have maybe a dozen people, if that, use it...