I think it would be interesting and useful to be able to blend alpha and paint towards a particular level of alpha as though it were a color.
It would allow more direct and precise control of alpha in a given layer without the need for a separate masking layer, cutting down time in certain workloads, since you would no longer have to switch between the two layers and work on them separately.
It would also allow for very quick use of layers as a sort of underlying pattern by allowing a brush to target a particular amount of a lower layer to show, making the layer essentially the brush's 4th color, while still working on the other 3 colors of the primary layer as normal.

I can see this being useful in sprite, texture, and general graphics work as a timesaver, as well as something fun to play around with as an artist.

I'm not quite solid on how it would be best to implement, but I feel like having a brush option that tells the brush to treat alpha as a color might be the simplest and most convenient way for an artist to use.