I was beginning to worry Elemental was going to be too derivative; then, there were spell books.  Wonderful Idea.  No one can have it all.  Specialization exists.  Each game is unique.  Also the use of shards helps play balance and give armies a reason to fight outside the city.

So why stop there.  You can apply the same meta-game concept to unit equipment.  Blueprints rather than spell books.  You specialize early during sovereign creation by selecting a default set of prints.  Some are found in the world or through quests.  Default prints can be researched to provide balance.  However, the best items should require discovery and control of a world asset to manufacture (e.g. a mine).  Again, no one can have it all, each game is unique, and there is another reason to fight outside the city; if only, to deny your opponent.