I think the general plan of items having x amount of sockets, which can be filled with various different objects that provide attribute buffs, is a proven and popular idea.

How the rules are designed, and the terminology used, is still up for debate. Crystal sockets, or just a more abstract Item Mod + Mod Slot, would be fine. Moon stones is nice too, actually. There are plenty of moons on this game

EDIT: Another thing is being able to enhance the mod/crystal/moonstone before using it is a good plan too. For the Crystals, the plan was to have Crystal Growing Kits, where you could take one or more lesser crystals, and, along with some other ingredients, create a more powerful crystal from them.

When you have something more specific like moon stones or crystals, this makes more sense. I guess it would be harder to do something like that with "item mods". It depends upon the naming convention of the mods, I suppose.