Well, I think a griffin (or, better yet, a hippogriff) would be terribly revealing because they are both symbols of Christ. The hippogriff would be more revealing because of Harry's (and Sirius's) association with Buckbeak, but either animal, to me, would say a lot about Snape's essential nature.

However, I am one of those who has become convinced that, whatever Snape's patronus was before, it's a phoenix now. That's why Rowling was at such pains to teach us that a shock or strong emotions can cause a change in someone's patronus. There are going to be important messages coming to the kids from a phoenix patronus - and it will be Snape. That's my guess, anyway. What do you think?

(I'm almost convinced about the Griffin, but not quite - originally, I thought his patronus would be dog, fox or wolf, which also suit his nature but which - other than pointing out his strong similarity to Sirius - reveal nothing at all. Yes, a Lily patronus would be a shocker, but somehow I don't think it's likely. Since all the order members would have seen it, wouldn't they? So how could they have any doubts as to his *real* motivation for abandoning Voldemort? And why wouldn't someone mention it to Harry?)

Love your icon, BTW. That's a great picture!