Not a lot of uses for moon rock outside of people obsessively swapping characters and crafting a million moon dials. Moon rock is proven ingame to be harder and more resistant to damage than natural rock (moon wall vs rock wall). Would be a great end game hammer.
Klei can also re-use the current hammer model and some skins by turning some areas blue (its a blue rock).
How powerful should it be? Same durability as a normal hammer but it destroys anything with a single swing. Great when placing large amounts of walls and you mess up. Its not like its op to one shot thelucite walls and other stuff in the ruins.