I never saw Dr. Conners as a legit threat as a villain. Sure, he could kill Spidey, but I never felt like he was big enough to base a movie on. Besides, his story is too similar to Green Goblin (who has no similarities to the Joker, BTW). Lizard is another scientist that becomes a victim of his own technology and transforms into a mindless villain. Of course, he and Peter Parker have some sort of relationship before (depending on the writer, The Lzard is one of Peter's professors). I guess the big guns have already been used.
I guess Lizard will provide a good test for Spider-Man since he's just getting his new powers and just starting out. We'll see.