I'd love to see tri-planar or blended box texture mapping added to RPR. It's a very convenient, fast, hassle-free alternative to UV unwrapping, by projecting a texture from three axes (tri-planar) or by box mapping (six sides), with the added option of blending the seams.
Cycles offers blended box mapping in its image texture node, and Keyshot offers tri-planar mapping.
My second big wish for RPR is vertex color support. Until that is implemented, scenes exported from ZBrush can't be rendered without going through tedious Polypaint to texture conversion steps. And vertex maps are also very useful as texture masks, among other things.