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I think that it's all but confirmed that it's going to be Electro in the second-one, Blue Boy, however, he might be seen towards the conclusion as a teaser for the third installment in the series. I don't know, he could certainly be interesting, you gotta know they'll be picking Osbourne sooner or later, but they might hold-off until the fourth movie. The thing is that I don't know if they'll be going to have Rhino be the flag-shipped character of the movie, or have him be a secondary character like Sandman in Spider-Man 3.
Rhino always struck me as more of a second to primary kind-of guy, the guy there for destruction, and not for anything very convoluted or intelligent, which is a primary necessity for any horror movie. Although, then again, I said the same thing about Bane prior to The Dark Knight Rises. However, at least with Bane, in the comic-books, he was carved out as a bad-ass, it was just Batman & Robin where he blew, whereas Rhino has always kind-of been just out there. I've been surprised before though, we'll see.
One thing that I am curious about is whether or not they're going to have Osbourne be similar to how he was in the first movie as Green Goblin, the animated series, or maybe something kind-of like what they did with Shattered Dimensions. For those that don't know, he was basically a relatively large, and fiendish fellow, more psychotic and darkly-based, similar to The Joker from The Dark Knight, but not with the comedic undertones, he was a more apathetic kind-of crazy.