You know what would be a cool thing to happen in SPN?
...oh wait, first have to say - have not seen new ep (not long though, EEE!), and NO SPOILERS PLEASE! Also, this will never happen I'm sure but it would be pretty frickin' awesome anyway, I think.
Remember Croatoan? What if the demons that are not on Lilith's side unleashed the virus en masse, knowing that Sam is immune to it, in an effort to force him to lead them. They could be all, 'Ha, now you have to fight all these demon!zombie people but we'll help you if you lead us and defeat Lilith and fulfil your destiny and incidentally, we'll tear up Dean's death contract while we're at it.'
That would be cool. Lots of desperate fighting against infected people and Sam forced into being what he hates and Dean feeling guilty for being alive (again) but still grateful for not being in Hell etc and then the fourth season will be ONE EPIC BATTLE!
I mean, why tell us about a virus Sam is immune to if they're not going to use it?