Tapered mask feathering
Ability to taper a mask feather so that one end can be more feathered than another. Real world example: Rotoscoping a moving character and being able to mask a limb that has more motion blur at one end from another.
I get there are other ways of achieving this same effect, and even other methods of getting a better cut out, but the flexibility would be fantastic, and could apply to a number of other scenarios. Think of the mask feather controls from Premiere.