Here's another one: An "explore" mode that holds altitude while still controlling the vertical movement of the camera. This way you can fly forward and turn while looking up or down without climbing or descending.
The FPV is already capable of sort of this because you can adjust the gimbal angle with the little slider dynamically in flight, achieving the desired result, but not as smoothly/easily as with an altitude lock, and the normal tilt forward of the entire stick adjusts view angle, as I'd like.