here's some predictions and rumors about it...
http://www.slashfilm.com/2007/05/06/...ains-revealed/
http://www.slashfilm.com/article.php...man4-confirmed
http://www.imdb.com/title/tt0948470/
And ahh, wikipedia... so reliable... yet... is it?
It gives some background in the production of the movie... and if you go to the bottom, it gives some feasible insights on the intents on Raimi and the main cast characters... And it states that even Dylan Baker (Dr. Curt Conners) agreed that the Lizard woul make a great add-in... and Raimi also agreed to "ponder" about it.
But all the details are withheld here, so take a look.
http://en.wikipedia.org/wiki/Spider-Man_film_series