After Will Smith's I Am Legend came out the studio wanted to get a sequel into development. They wanted Smith to star in the sequel, the only problem was (spoiler!) his character died at the end of the first movie. Screenwriter Akiva Goldsman was hired to figure out how to make it happen.
In a recent interview with io9, Goldsman shared several of the ideas that he came up with for the film that thankfully never happened. None of the ideas seem like they could have really worked. When asked about the sequel, this is what he said,
Oh no no, [there was] never a movie. I mean, we wrote a prequel [and] a sequel. We had a really interesting prequel, which was the first outbreak of the virus, during a Thanksgiving Day parade, which was awesome. We did a really interesting prequel that [took place] later. Which was right after the first... really right when the population of of humans became pretty decimated. And it was this sort of trek to Washington. Which included a dark seeker elephant that had broken out of the zoo.
Then we did a sequel, that started with Neville again — and you realized that he was cloned. We've tried every which way. In fact if you're available you could be in the next movie. It will never happen but we really enjoyed trying to make it happen.
The first movie was based on the 1954 novel by Richard Matheson, and Vincent Price starred in the 1964 adaptation The Last Man on Earth, which in my opinion is better than Smith's film.