I think it worked well in Spiderman 3. Killing venom was probably a mistake though considering he can't easily be brought back into the script.

As for an entirely new movie with him, it would have to be separate. Bad thing is that the plots can't really coincide. Venom was created and killed within the course of Spiderman 3, which ends up at no more than a few days in plot timeline.

They would have to think of a new way to introduce him, or a way to revive him somehow. Both of which I think will not go over well.