Shards and Key Resources
I was just reading a post in the PC gaming section about Warlords II and Warlords Battlecry. This caused me to remember the beauty of Battlecry's resource capture model. In Battlecry, you had to move a special unit near the resources and channel for a while to gain control of mines. Because the current owner could see this action (i.e. fog of war lifted due to resource ownership) they had ample time to respond to this hostile takeover.
I'm not suggesting Elemental directly copy this mechanic. However, if the action used to takeover an enemy shard or resource required several turns the defending player could respond. This would remove one of my biggest compliants with AOW where resources would constantly flip-flop. As I've stated in other threads, as long as units have upkeep players will not defend resources by garrisoning troops nearby. Therefore, it takes time to respond.