It's been speculated upon for months but, in an interview with Total Film, Electro himself, Jamie Foxx, seems to confirm that, at the very least, the villainous team known as the Sinister Six HAS been a topic of discussion for future Spider-Man films.
The Sinister Six is an assemblage of Spidey's biggest foes and a staple of his comics over the years. It is a very ambitious undertaking to have them all in one film but would be an absolutely awesome achievement if done well. I loved reading Spidey comics as a kid. I have as many Spidey comics as anything other than X-Men. I admittedly have lost interest over the years but I liked the Andrew Garfield version ok and am looking forward to the sequel. This should take any sting out of the "death" of Electro in the sequel if that's the route they take.