Rhino
Who should play him? Paul Giamatti, because it's Paul Giamatti. And he's already established as the character. I guess you could recast him if need be, but why would you want to? It's Paul Giamatti. Playing Rhino! Anyone else would be a major letdown.
Why would/should they be included? He could be included in the lineup since he'll already be part of this movie universe. But it seems he's only in the new film as a bit of fan-service. Giamatti is a talented actor who doesn't seem like the type to get tied down to a major, recurring blockbuster role. Kind of like Scarecrow or Zsasz in The Dark Knight Trilogy, he's there but he's not prominent enough or has enough of a beef with Spidey to deserve membership. He shouldn't be included, but considering he's already shown up there's a chance he will be included- if he's in their for reinforcement, we have a slightly less crowded movie where we can focus on Spiderman and only 5 of the six antagonists.
Likelihood: 3 out of 5 He's a renowned Spiderman character, but not exactly a great one. There's a chance he'll be in their as muscle, but with two other films to set up other, more interesting villains, why would he be included? Especially is Marc Webb's comments are true and he's only in a few scenes. He's got a better chance than villains and character we haven't met yet, like characters you're going to meet further down this list, but then out of villains already established in this series, he has the smallest chance of reappearing.