Decal layer (like Fill with move, rotate, scale)
Actually, adding move, rotate, scale support for the fill layer, would do the trick IMO.
All this hooked up in the 2D viewport in a similar way as you place the the stencil mask.
With proper masking that can be used for decals.
@contact_32, the big advantage of a fill layer over a stencil that helps you paint over, is that you can tweak the parameters further later.
Also having a way to deform it with a lattice and control points would be awesome too.
The ability to transform the decal with a grid like will be great too, like ion zbrush, mari, mudbox for projection painting.
I added vote to this, however, in reality it's already added, you add a regular layer and use a stencil as is shown in the tutorials, I don't see any difference, and you can name the layer Decal.
Ability to use non-square texture images would be helpful.