I'm really pleased with Krita's texture painting ability's yet many concept artist also use displacement map filters or grid warps to create more 3d like textures on artwork. Some example links http://youtu.be/FpYwxPE9J54 http://www.deviantart.com/art/Displacement-Maps-Tutorial-PS-102175264 sure these links are based on photos, yet displacement maps could be used on paintings as well to provide faster work flow in adding things like scales to dragons ect, displacement maps can have some interesting possibilities.
I have another idea though that I'm not sure if its possible, that idea is that instead of a filter that you apply to two prepared layers, there would instead be a way to actually paint displacement textures onto a painting, creating a more volumetric like feel to texture painting, the painting tool would create a sample layer that exists in greyscale to calculate displacement like the filter yet it would not displace another layer until it is painted on with textured brush allowing artist to paint different textures in a Psuedo 3d like manner in real time, if possible that would be an awesome feature that so far I haven't seen yet in other programs. Just a suggestion not sure if its possible.

hm... you can actually sorta do this.

Assuming you have the ideal displacement texture.
Make a new image. Add a filter layer(I think it's called phong-model... something, I'm sorry you'll have to look for it, but it renders greyscale as bumpmaps in 3d).
Now on the lower paint layer start painting with you displacement texture.

Edit: I think this program you're thinking of does exist, it's called Zbrush. It started out as a 2.5d painting program

Hmm phong-model I'll have to check that out to see its possibilities thank you TheraHedwig for the response, wow never knew that about Zbrush I knew you could sculpt and color in real 3d never knew about the 2.5d part. Thanks for sharing your knowledge with me. I always enjoy learning something new.

That's good

The filter is under Filters>Map>PhongBumpmap.

http://img.ie/6ens4.png

Here's an example, with filter-mask instead of a filter layer for extra control, but see for yourself how you'll put them together.

Wow that'd definitely an interesting effect. I can see how the bump map adds some height. lol when I can stop procrastinating and my mind isn't distracted I'll give it ago.