Here are my top feature requests...
1. Higher Resolution (at least 8k for max size)
2. Color pick from anywhere (viewport, UI, desktop) & modeless color selector
3. Clone tool
4. Symmetry
5. More intuitive FILL tools for both color and material.
6. Edge Masking based on normals falloff (
http://youtu.be/cZXo3ywEzLI)
7. Cavity/AO masking
8. UDIM
The other thing I would love to see doesn't really fit as a bullet point list item. What I would love to have is a way to set the properties of the object's materials separately from the painting process. The workflow I would prefer for creating texture maps is to paint each one (diff, spec, bump, etc) and not paint materials with the brushes. The shader would be controlled via a UI panel where you adjust the basic parameters and the contribution of each map type. Make sense? This would make SP more suited for professional VFX work and a great companion for Zbrush. Check out how Mari handles this.