Here's a few basic ones that nobody seems to have mentioned yet:
Tiling option for diffuse textures (what we in SL know as textures right now) with UV mapping outside the normal space. Default would be 'Repeat' (normal SL behavior), with 'Extend' (extends edge color past boundaries) and 'Flip' (mirrors texture past boundaries), possibly with separate Horizontal and Vertical options.
'Sprite' option. This would need documented, but would cause the geometry facing a certain direction (say, positive X, like avatar meshes) to render similar to a static, unfading particle. Unlike a particle, though, this could be textured like a prim or mesh, allowing for animation, or other prim face effects.
Change Shiny (old style) to Reflection, adding a space for a texture to be used, instead of the procedurally-generated reflection texture currently used. Add the 0-1.0 or percentage option to replace Low, Medium and High.
Now, this other one, is a little more complex, so allow me to explain.
I'd like to have a variable-width 'line' geometry available as a per-'face' (in SL terms, meaning a mesh material) option. It would be generated by the hard/sharp/unsmoothed edges in any given mesh or prim, and be viewable from all angles (when selecting the 'Line' option, the thickness would be half the chosen width on each side of the edge). The face/material would also accept UV mapping for as much texture area is shown.