But back on track here: there was three acceptable scenarios in my opinion:
1. Shirley died. Simply put, Shirley was too broken a human being at that point. She had lost her father, the one man she loves is responsible, and she just murdered someone herself. There was no turning back. She was done. Personally, I liked this route because it represents both the loss of innocence and consequences of Lelouch's actions. Lelouch's goal is to destroy the world and rebuild it for Nunally. Yet in the process, he often loses sight of the fact that what his doing is also destroying innocent people. And having it be someone very close to him. Of course, this is the same guy who accidentally Geass'd Euphy into murdering a bunch of Japanese, and after a little bit of angsting, decided "F*** it, let's use this to our advantage." So I don't think Lulu would have any real regrets. But it'd be pretty good character development to hang over him for a while, and it might even explain why he deals with Euphy relatively calmly (he's spent time hardening himself since Shirley's death).
2. One of the two scenarios described above. At least then returning to the status quo would make sense rather than the crap we got.
3. Shirley actually becomes a completely different person. To reflect what she's been through and how broken she's become, Shirley becoming a new character makes a lot of sense. What type of character, I don't know really. Perhaps she actually becomes an enemy of the Black Knights and Zero.
Rant over.
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TheRock ~ Death By Misadventure, Not Suicide
"Would've liked to see that." - Ayuyu on my boner