Since weapon based skills are no longer universal and are instead specific per weapon type, the choice of weapon determines what attack skill gems you will be using. Say I picked a particular weapon type: would there be a complete suite of fire, cold, lightning, chaos, and physical spear skill gems for me to select from and make a build around? Or would some elements be unaccounted for depending on the weapon type?
For example, we have seen some spear skill gems of varying elements, but I don't think we've seen any associated with chaos. If the answer to this question is yes, then we'd expect there to be at least one chaos spear skill gem that has not been revealed yet.