I wouldn't be surprised if the final episode does have Lucifer rising, that way they could have Dean being the one to break the first seal, and Sam breaking the last seal.

Oh, that's a very cool idea! I did think perhaps killing Lilith was the last seal, but I hadn't thought about Dean and Sam as the Alpha and Omega, so to speak, before. And a post-apocalyptic landscape for season 5 would certainly be an interesting one.