1. Recast
If we are being perfectly honest with ourselves, we know this is the way that Marvel will most likely play it when actors leave their roles. Kevin Feige has even said that they would go the route of James Bond and just re-cast actors as needs be. They already have a history of this with every iteration of the Hulk. The only problem I really see with this is that no actor has ever embodied a superhero so completely and having another actor just simply step into those shoes is going to be jarring to say the least. Robert Downey Jr, especially in his very troubled past, so on the money in terms of exploring what Tony Stark is. Eccentric, charismatic and very troubled. We are almost to the point now that this incarnation of Tony Stark is now burned into our psyche that another could feel like an imitation of RDJ and if not, it won't feel like Tony Stark. I am not saying that another actor couldn't take on the role and do really well in it. In fact I would be the first to support the new actor. It would be fun to see a fresh face take on that character. My worries are purely for the fan base. I think there is a ton of fans who might be completely turned off by a new Tony Stark. Plus I do think that there would be a missed story opportunity.