I have small experience with UAVs, and absolutely no any experience with FPV, FPV-cameras, interfaces, etc... Are you still flying on NTSC or PAL? 
That's why I have some question to you and community:
1. What do you think about this addon? Is it worth doing?
2. What about total weight and dimentions?
3. What kind of interfaces FPV pilots use nowadays? Is AV enough? What is the output format (NTSC/PAL/or...)?
4. What else is reasonable to add to the board?
I have experience.
Focusing - not needed for flying, but needed for ground (for example, a robot for surveying communications) drones.
Need Ingress Protection (IP).
The following interfaces are required:
- AV (PAL25, NTSC30) - yes, this is relevant, there is no delay;
- USB (for Raspberry Pi);
- miniHDMI (720p60);
- PWM / UART / PPM - switching palette, focus (if implemented) and calibration (on, off, automatic).
For the broad masses of users, PWM / PPM is relevant.
In my projects, I like to implement such interactions on UART:
- 2 bytes for synchronization;
- information byte settings flags (palette, calibration, autofocus);
- information byte 255 gradations of focus;
- the last byte is the modulo sum of two information bytes.
Power supply 5v (required) or 8-60v (optional).
Interchangeable back panel, only with required interfaces. For example coaxial sma/mcx for AV and 4 pin power/uart.