I really love the Nemesis idea however it seems it has so much potential but that is not being utilized fully.
Perhaps in the Expansion we will have additional reputation level (Exalted?).
What if once reaching highest level of Enemy faction rep, there will be 20% chance that every time the Player uses a Rift he is to be ambushed by Random faction Nemesis + 4-5 of that factions Hero`es.
It will give real meaning to the word Nemesis and it will provide the player constant feeling of being in danger of getting ganked by the enemy.
It will also remove the need for the player to chase where the Nemesis has spawned - imo it makes no sense - if the Nemesis is unleashed it should chase the player death or die trying.