Well here's a thought to the HNM idea, make something that encourages co-operation between linkshells. Say you have a HNM, a BIG HNM. In order to take it down however you have to apply damage at different points of the mob simultaneously or keep a mob from defending his weak spot.

For instance Group 1 (G1) has to apply enough hate from the front while Group 2 (G2) attacks the weak point to do damage. If G1 cannot keep the attention of the HNM from G2, the HNM can retaliate against those at his weak spot seriously damaging moves on top of heal himself through high-regen. I guess you could call this "Group Focus" where the mobs hate is not only based on the person with the highest hate, but the group with the highest collective hate to cement the mobs attention.

Once the HNM is finally defeated, a chest spawns for each player or group that actively participated in the battle that has a chance of dropping gear, similar to Campaign rewards in XI you only get something if you put enough effort into it.

Anyways that's just an example, and you can easily apply this kind of setting to more than two groups/alliances as long as the HNM was difficult enough to warrant shells trying to help each other out. And it would encourage more friendly linkshell communications with one another.