Regarding your good ideas. They don’t seem really feasible right now for such a small company.
Offering different module variants like e.g. with/without headphone jack means a lower quantity of every module, which comes together with higher prices.
If they sell the phones as a kind of construction kit, it would come without OS.
As it is now, the OS has to be tailored to the hardware. Therefore every possible hardware combination would need an adjusted OS. If you get a kit, you would need to install the right one after assembling the modules.
And if the phone is manufactured to order, it will become really expensive (like all tailored stuff).
I doubt, that there would be too many people willing to pay even more than now.
Maybe, once Fairphone is a bigger company, they can take steps in that direction.