Venom
Who should play him? If you want a bulked-up Eddie Brock, then my dream casting is Tom Hardy (Bronson infected by the symbiote!), but that is unlikely after he's played Bane. James Badge Dale could be a good choice, but it's unlikely he'd tie himself to another franchise after Iron Man.If we're going for the Mac Gargan interpretation of a weedy, Topher Grace-type (Please God No!), then perhaps Sam Rockwell would inject some vibrancy into Venom?
Why would/should they be included? Because Venom is awesome and wasn't done justice in Spiderman 3, although who's to blame for that is unclear- Raimi? Topher Grace? The execs who forced Venom into the movie to please the fanboys? Seriously, there's every chance that the Symbiote could be a plot point, and you can't have the Symbiote without Venom. He wasn't prominent enough in Spiderman 3 that including him in these new films would be sacrilege, and he's just the right sort of major player who could fit in. The Symbiote is the really villain, if you think about it. There's so many directions it could be taken. Plus, if Venom became the last piece of the Sinister Six puzzle to fall into place, we may just get the Venom spinoff that's been threatened for so long!
Likelihood: 3 out of 5 He's been alluded to in set photos, a venom solo movie has been in development forever, and there was that rumour going round about Jim Carrey playing Cletus Kasady, which seems utter bunkum. Basically, bizarre internet speculation aside, it still seems very likely. There's a demand for Venom from both fans and the studio, for various reasons. And he's cool and still popular despite a minor appearance in Spiderman 3. There's room for him to return, and many seem to be clamouring for him. I'd say out of any major villain/potential member who hasn't be suggested seriously yet in the films, Venom has the best and most likely chance of showing up.