This is a bare bones idea but...
The primary antagonist will be my boy, Levankua, and will climax with a fight to the death between him and Tempest. I want to do this after the hell arc.
Here's the skeleton of the plot:
1. Levankua's minions sneak into the Soul Society and steal some technology. This provokes a responce from the Shinigami. A group of Tempest and several 3rd seats and Lts will hunt down and confront Levankua. After some fighting Levankua's group flees into an unsupervised part of Hueco Mundo temporarily.
2. Less than a week later, Levankua and his men have once again provoked the Shinigami. The Primera Espada decides that an Espada should deal with the traitor. Both a captain and an espada, and possibly a daga, will fight and destroy Levankua's group, leaving only Levankua to escape after recieving injury.
3. Unable to hide in Hueco Mundo, Levankua is forced to take refuge in the human world. Tempest locates him and fights him for the last time.
Please...this could really use some polish like a dirty car needs a wash