The ability to add layer masks to a photo with separate editing controls for color, light and curves. Or if you wanted to make it epic, allow a separate layer mask that allows for preset integration with opacity control.
Masks can be added via “Local Adjustments”. See the User Guide in the Support section. The mask can be viewed with the M key. However, I agree, an opacity slider would be nice.
Yes the local adjustment does provide a nominal effect of a layer mask. It does not perform a true singular mask without the use of a brush or gradient. Additionally, the local effect controls for color are very unpredictable (from a user standpoint). The ability to add an adjustable layer mask that incorporates all of the standard palette options or a solid color with opacity would be a major game changer for many that want to add color tone without having to use the split-tone feature (which also behaves very odd in it’s current state, and has a very limited choice of colors).
Now, if beggars could choose and you could make a solid true layer mask that could overlay film effects… CaptureOne might loose some sleep at night