With the advent of CGI in the past 15 years comic book films have really been able to capitalize on the technology like no other genre. The mixture of live action and CGI has been blended to near seamless perfection. Yes, fans know that most of the imagery and even some characters are completely computer rendered, but we have learned to accept it and not be taken out of the story by it.
The first near perfect rendering was in the "Lord of the Rings" trilogy with the Gollum character. Voiced and motion capture performed by Andy Serkis, the character really came to life and was so believable some were calling for Serkis to get an Oscar nomination for supporting actor. Serkis went on to play another mo-cap role in "King Kong" and even though we knew there was no ape that large interacting with Adrian Brody and Naomi Watts we had no trouble settling in and following along. Recently Serkis played Ceasar in the [Dawn of the Planet of the Apes](movie:322904) and again, the CGI is flawless.
When it comes to human roles rendered in CGI I will skip "Polar Express" and "The Adventures of Tintin" because I feel those films were aiming for a fantasy tinged look. However "Beowulf" which...even though it is a fantasy story and had some non-human characters in it, really set out to bring very humanistic performances out of CGI renderings.
Comic book themed films are already very CGI heavy. Many scenes have rendering in them and the audience is not aware of it. Many shots of Iron Man's armor in the MCU films are actually CGI, not someone in a costume. We know of at least one Avenger that is full CGI when onscreen and that is a transformed Hulk.
As the Marvel universe in particular grows and expands we have to expect more CGI characters to be introduced. In the latest Marvel film [Guardians of the Galaxy](movie:424073) we had two main and one supporting character that were completely rendered.
With all of the worry and speculation about actors contracts and how they will age during the years to come, mixed in with the fact that it, so far, is about two years between appearances on screen for many of the main actors. It will take a long time to tell some of the big stories that are lurking in those pages of 70 odd years of comics. With improving CGI I think RDJ and crew could be playing these characters for years to come.
This is not an unprecedented thought. Many are predicting that soon advanced CGI will be able to bring deceased actors back to life on screen to interact with modern actors. In fact the late Audrey Hepburn was recently featured in a television commercial for Dove chocolate that had some people floored and some really creeped out.
Let's also take into account the cut scenes for todays video games. How often do you hear people say "Why don't they just make a movie like this?" I mean sure the budget would have to be pumped up and the action mo-capped, but it will still be quite a bit less expensive than todays blockbusters.
Maybe not now but surely in about ten years or so. I can't see how this won't be the norm for superhero films. Think about it...when we get to the point where Marvel, Fox and Sony say "Okay, lets do that 'Civil War' film the fans have been screaming about for the last 15 years" won't everyone want to see Hugh Jackman's Wolverine go up against The Hulk? This may be the only way we are ever going to see it.
UPDATE: Someone directed me to this footage, it's not at all official Marvel product, but it hints at what could be...
I see the possibilities..