Snape and Love.
(now that's a surprise, isn't it?)
We've had Lily as a theme already, but I don't want you to only deal with Lily. Take love in all its variations.
What and whom did/does Severus love?
Who loves him (and we all know that the Underground does, so I don't want to see posts with 'I do' ;) )?
This:
http://en.wikipedia.org/wiki/Love will give you an idea.