Now, I know this has been the great debate. I was on youtube and I decided to watch that credits scene again and I think I came across the answer that never occured to me and it invloves Dr.Curt Connors A.K.A The Lizard.
So, what is my big idea? Well, no one explained whether the antidote fully got rid of the reptillian foe. Just like in the comics, Dr.Connors is never fully cured. The man is possibly a hallucination of Connors mind because The Lizard serum never fully wore off. It makes sense. At least to me anyways. I am not saying he's the villain in the sequel, despite the fact that I LOVED his portrayal in the movie. He should not be the villain in the sequel. Although, he could play a pivotal part in it.
What about you? Comment below with your thoughts.