Some type of "an indirect reactive aggro system" could be cool. An aggro not only based on players range to a particular monster, or based on initial pull, but an aggro based on travelling monsters passing by other monsters, and perhaps based on monster type as well.
So if one bear notices another bear from it's "tribe" in danger (in combat), it would react even if it does not notice the player that's attacking that other bear. It would first run towards the bear who needed help, and then check whats up. At that point if bear saw a player attacking a bear he would react to the player. A bear would not instantly run to a player that has aggro, because this might look foolish, because that other bear might not notice player instantly from his original position.
On the other hand if wolf saw a bear needing help, he might not react, and he might ignore the bear (he might even attack the bear to exploit weakness if bear was it's enemy), unless he perhaps saw the player afterwards when he reached the bear and then he started to decide if to attack the player because he wanted to attack the player or keep attacking the bear.
Some more complex aggro system would be cool if designed like that.
Of course game still needs "aggro reset range", because players pulling a whole forest to the city would also likely not be desired.
Mobs would stick to their own area, perhaps and "aggro reset range" would not be fixed to a particular distance, but would be tied to a certain zone. So a bear would chase you while he is in his own forest (home) but when you reached the meadow, or a part which bear does not consider his home anymore, bear would give up, regardless if that was 30m, or 250m.
In any way, I'm also hoping that we will see something more than a standard MMO aggro system.