I am aware of the current use of the Ecumenical Key; as it allows for immediate entry into any of the 4 GWD boss rooms, without requiring the certain kill count. However, I propose that it have the added functionality of creating a single instance within one of the 4 boss rooms.
Since the NPC's located within the GWD are rarely thoroughly grinded, unless it is the Spiritual mages for Dragon Boots, this gives more reason to grind these NPC's.
I believe that this addition would benefit most players, more so for each type of Ironman. Finally, since the drop rate for one of these keys is typically around 1/60, it would be a fair addition.