Temple of war (artifacts) and or Sanctum of Shards (crystals)
Either make it possible to upgrade Temple of War to apply more artefacts
maybe 1 more artefact at level 10 then 1 more at level 20?
or which I think would be better and promote activity too ; make it possible to upgrade the existing artefacts by combing them same as the crystals in the sanctum of shards.
So that we get artefacts with a level (and slight increase in their power)
also increase the level capacity of the Sanctum of Shards and with that the level cap of the crystals.. a new Rod.