For the actual design perhaps starting with something like a stripped NF (but perhaps bigger) for a springer, and a decent metal compressed air tank for the tank blasters (tankers?).
I would think designing both mechanisms to be of comparable size, so that the internals could be different but fit in the same shells, would be ideal.
Or perhaps, have an attachable stock for tankers, that is the air tank. For example, soda stream has 40L/60L/100L* CO2 tanks.
Also if you want to go all the way, it shouldn't be hard to design the springer, so that you can use a motor in it for auto, or take out the motor kit and use it as a semi-auto.
*Currently have a plan running for a Raider conversion to allow a 100L tank and a rotating tubular magazine. Just have to actually start doing something about it....