Now...now, don't be rude to Kallen.
She was obviously too good for Lelouch and he knew it.
That's why he rejected her at the end fo the series.
While CC on the other hand, was as messed up as he was.
I think Kallen's sense of justice, her bravado, and certainly her "purity" (if any character in Geass can be described that way) was better suited to someone else.
Someone who had lost his true love and could appreciate a fine woman like Kallen.
I know few CG fans (especially the Kalulus) like the idea of this pairing, but for me it holds some merit.
Here's a pick of the pairs I wish Sunrise had used at the end of Geass:
