Personally, I think that it would be best for balancing to not have an aa that affects the entire group, those spells tend to be extremely powerful when used in certain circumstances, and to be honest, necros are already extremely powerful in most situations. The problem with leechstorm is that it's a long recast for something that only applies to a certain situation. 5 mobs attacking you? Leechstorm. That's about it as far as usefulness of this aa goes. What I think needs to happen is that leechstorm be changed to something that further increases the necros ability to do what they already do, which leads me to a few suggestions:
1.) Mana conversion. Necros are the kings of mana regeneration, but at a cost, for every point of mana comes a point of damage. The aa could be changed to:
Lichking
+135 mana, -300 hp per tick, lasts for 6 ticks. Stacks with all Lich Lines
2.) DoT damage. Necros are also the kings of DoT damage. Since it would not be possible to have an ultimate blast for DoT's that is passive, perhaps an active AA that can help with burst dps in certain spots, say something like:
Destructive Force of Marlow
Increase DoT spell damage by 25% for 3 ticks, drain 1500 life from necro.
3.) Lifetaps. Necros get the best lifetaps, both in DD and DoT form. How about something like this:
Mortal Knowledge
All lifetap spells do 10% more damage and heal for 20% more, lasts for 4 minutes.