If you are curious as to what goes into designing a joystick, have a look at this 2014 article from the International Journal of Engineering Research & Technology (IJERT): Design and Development of Special Purpose Joystick
It is not specifically for a flight joystick but does give an idea of what would be required to design a new joystick for the Flight Simulator market.
If you are interested in market trends for gaming controllers, have a look at this article. There is an interesting chart that shows offerings vs. strategy.
And if you are thinking of starting a company to build flight peripherals & compete with the current players ( 
), have a a look at this article: How Much Does It Cost to Develop and Manufacture a New Electronic Product
Enjoy!