The basic idea is to perform a PID (or maybe just a PI or just an I) control loop in opamps. Then use the analog values to control a PWM generator to drive the motors. The final form is aimed to be 50x50mm or less. Powered by the original mini Li-Po pack from the helicopters, it should be entertaining for a few mins.
As this is purely an exercise in analogue electronics, it makes sense to focus on the control loop. After much thought, it seems easiest to make independent drive modules that will maintain a certain parameter such as altitude or velocity. I hope to add a purely analogue FM controller if the project persists.