I'm an underwater videographer - a relatively tiny segment of the user base - so I doubt I'll get many of my wishlist items but here are a few of mine:
1) Better UI, ability to navigate all settings with buttons (through UW housing) on the color back screen, not the tiny monochrome front screen. We could do this on the Hero4.
2) 1" sensor, probably not gonna happen in an action cam though.
3) Custom white balance.
4) Lockable if not manual focus.
5) Linear mode available in all resolutions and frame rates.
6) Wireless charging to reduce need to open the UW housing as often. This is important for several reasons. One, every time you open the housing is a risk of making a mistake (hair on o-ring) to cause a catastrophic flood. Two, open housing in hot humid tropics then dropping into cool water causes condensation and fogging, beyond the ability of anti-fog inserts.
7) Include metadata in the video file to store camera settings used. Frame rate and resolution is easy to determine but what about Filed Of View, ProTune, White Balance, iSO, Shutter, etc.
8) Depth information, like Paralenz camera. Now I'm really dreaming so maybe this could be an add-on for the housing.