title_body: Can I re-flash the Hubsan fpv1 controller with built-in fpv screen? the only radio transmitters, also called remote controllers, that embed a fpv screen, for seeing what the drone sees, near the joysticks, instead of having to get obstructed vision by goggles,
  that I found, are :
  
  https://www.youtube.com/watch?v=df0Cvv3T69Y called fpv1 (i'm not interested into h7000 as it runs android and requires wifi rather than RC) 
  https://www.youtube.com/watch?v=XBbptOuXXdA called fpv2 
  
  However they only seem to be able to bind Hubsan drones...
  
  Would it exist a way to flash them in order to make them run openTX firmware, or anything that could make them become universal controllers?
  
  Alternatively, what parts from Hubsan should I buy if I want to put in my own custom drone build and be compatible with the DIY ecosystem ?
  
  Can I build from scratch a controller with arduinos so that I could use openTX, embed an fpv screen, while still getting decent range (&gt;5km) ?
upvoted_answer: I'll start by warning that you may have misconceptions that are leading you in the wrong direction. The reason that FPV screens built into transmitters are not more popular is that screens aren't bright enough to compete with sunlight. The primary function of goggles is to cut out bright reflections, so you can see the picture.
  Also, transmitters are comfortable when held down at your waist, which would make the picture on these transmitters too small. You'd need something the size of a laptop or tablet computer screen to work well at arms length. The picture from an FPV camera is pretty low quality by modern TV standards, and needs all the help it can get.
  The transmitters you link can't be flashed to run OpenTx. They just weren't designed to be modified like that. They may not even have enough flash memory or processing power, as OpenTx has vastly more features. Similarly Hubsan doesn't sell a stand-alone receiver for their transmitters.
  You probably could buy an OpenTx radio and transplant the electronics into one of these cases, but I think you'd have to find something clever to make their TV format screens display information that was designed for a dot-matrix screen. You could look at the boards used for OSD (On Screen Display) but I imagine it would get very technical.
  I think your best bet would be one of the many FPV monitors, mounted to the top of a normal transmitter - just be sure to stand in a shady spot.