My idea would be to make a Treasury building and add Artifact items for which players could fight constantly and that aren't resources.
Treasury - A new building
The Treasury would be the place to store Artifacts and it has a limit of holding maximum 3 type of artifacts.
Artifacts - Lootable items that need to be defended
Artifacts are items that have magical powers which give some benefits to players who hold these items.
-Each artifact is consisting of 3 pieces so a maximum of 9 pieces of 3 different artifacts can be stored in the Treasury.
-The more pieces you have from an artifact in your Treasury the higher benefit you get.
-A full set of 3 pieces from an artifact type gives the highest benefit.
-Artifacts have powers such as they decrease of 5 - 10% of all building costs or another one to speed up construction and research by 5 - 10%. Another one could increase the HP of all buildings by 5 - 10%.
-There are 3 different types of artifacts available.
Battle for Artifacts
The way to receive Artifacts is to attack other players' treasury and loot artifact pieces.
-Attackers can only loot a maximum of 1 piece per attack.
-Attacker doesn't know if defender has any artifacts in the treasury and if they have which type of artifact it is from
-After a successful looting attacker can decide to keep or leave an artifact piece
-If you keep an artifact piece from looting it will go to your treasury
-If you leave the looted piece it will go back to defender's Treasury
-Attacker can only loot an artifact if he has Treasury that is not full
-Until you own all 3 pieces from an artifact type in your Treasury you cannot keep an artifact piece of the same type after looting
I'm looking forward to hear what you think about this idea.

