I was thinking about such a design as the “ultimate” drone, so good to see somebody trying to get such a thing working. I was inspired by the Tilt drone and Avatar film, with independent servos for each arm. With linear servos (screw-thread style) maybe the arms attach directly onto them.
One issue you will have to consider is servo update speed, similar to another question posted here the other day. As you plan to leave your ESCs pretty much constant that should not be a problem with lower update speeds, or maybe you can find good digital servos supporting fast updates. No analog servos then.
To help you with some inspiration check this out (just fun, it’s not real): Prototype Quadrotor with Machine Gun! - YouTube
On the serious side that mock-up drone with “thrust vectoring” (not just variable pitch) is really more like one of the Avatar ships. In this case we would also need to tilt the motors side to side. That’s not possible with current/basic servos I think. It would need either a very large drone or better some kind of custom made on-arm gimbal.
The software to control such drones is going to be complex when used to full potential, so you can take advantage of the Navio/RasPi2 here. For example, imagine an FPV race where the rotors can tilt in opposing directions to air-brake on one side and power-through on the other, effectively going around a corner flat like the drone is on rails!
Hopefully you intend to keep it open source so we can all share and build on top of great ideas. It would be good to get frames like tilt, variable pitch and maybe later full thrust vectoring supported in APM and other autopilots. The guy who created Tilt said (in one of his announcement videos) he had great difficulty to customize one of the other autopilots (think it was CleanFlight or BaseFlight). To save time I’d check that out first for any tips.