The Civil War storyline is an odd choice. It was a big crossover, it did have Cap and IM at the forefront on opposite sides and in all fairness, if Evans/RDJ aren't going to stick around for a great deal longer (and they want to use the storyline) then they kinda have to do it sooner rather than later. Saying that, the whole arc is predicated on having a *huge* cast of heroes from all over the Marvel landscape.
The mutants famously sat it out (citing enough problems of their own), the Punisher and Spiderman were (while not strictly pivotal) certainly important in the roles they played. Without a large cast of heroes, the Civil War storyline simply lacks any teeth. In short, they need a *lot* more superheroes involved and taking sides for the storyline to matter, else it's just two people disagreeing with each other.
As an aside on the point about Spiderman, his "reveal" was a massive coup for Iron Man and pro-registration and his subsequent change of mind just as important. I can't remember the line exactly, but it was along the lines of "If even Spiderman, whose identity has always been the most closely guarded secret agrees...". But Spiderman can't be involved because he doesn't belong to Marvel.
40mill for a cameo...RDJ is the King of Hollywood. Not even Johnny Depp could command that fee.