Here are a few features applicable to rider's in the DA, TA, to an extent MA and film viewer I'd love to see implemented at some point:
"Riders"
1. see full working instrument data of plane they joined, especially flaps and throttle (rpm/manifold).
2. ability to "slave" rider's views to driver's so they can see how the driver is looking around. (in MA, too would be nice for SA training)
3. tracer visibility, right now the rider cannot see the driver firing if tracers are enabled.
4. external views while a rider and god's eye as mentioned.
5. ability to film - it used to be (still?) when a rider tried to film he may only get film of the tower.
I'm sure I forgot something, if i remember, i'll edit it in later.