HD is far from getting lighter, or cheaper, unfortunately Amimon owns teh main patents on the techno,logy required and they are less than motivated to make any real difference in the way its utilized. This is sad because I have run Connex almost exclusively since December 2016 and just recently started swtiching back. Without the manufacturer providign any support, havign any way to debug and repair ourselves, and other issues will stunt its adoption.
4" quads are a black hole of inefficiency. Remember that our aircrafts, quads, are terribly ineffective at energy transfer and efficiency. Its why multirotors in this form factor do not exist in real life. 4" is an area that unless we get wicked development in the 4" prop and motors wont ever come back.
2.5'-3" is much more efficient and less money to build, they are also just as fast if not arguably faster.
Coming down the pipe is the end of miniaturization and more about efficiency. Battery and Props are the main needs here. We are at the mercy of technology with batteries and props cost money to develop, so ultimately we are not left with much.
Prop and Motor combos is an area that needs huge reassesment and this will need repeatable autonomy in duplicating flight characteristics. This is an area to watch, as well as the continuing of consumerizing products and makign products aimed at novice and entry level buyers in ways that get them flying faster.
Digital protocols on ESCs and all that jazz of course.
