Kevin Feige, President of Marvel Studios, stated that Tony Stark's character would be similar to James Bond. Meaning, no reboots or new versions. The only way I see that working is if Tony Stark basically made "Tony Stark" the new "John Doe" for whoever dons the Iron Man armor. Who's worthy enough to replace RDJ in the red and yellow suir? Let's find out...
Robert Downey Jr. as "the original" Tony Stark
Plot: Stark, a man facing his own mortality, must accept the fact that he can no longer be Iron Man. In an effort to continue his celebrity and income he takes the trademark off of his name and gives it to the new Iron Man.
Let's take a look at some possible RDJ replacements now...
Leonardo DiCaptrio as "The New Tony Stark"
One of my favorite actors, I would cast him based off his past work, his level of intensity and playboy looks. Curiosity beckons me with what he could do with this role. It would actually be Leo's CBM debut.
Mark Wahlberg as a relative to Tony
Let's flip the script for a moment here. What if Tony had a son/cousin/nephew or whatever he didn't know about? That could make for some entertaining quips between Wahlberg and Downey Jr. for sure but the thing is Wahlberg could basically play anyone he wanted, whether it be a millionaire who purchased the suit fromo Tony under some conditions or whatever. The point is, Mark Wahlberg would definitely be an entertaining Iron Man.
Jack Huston as Cody Starks
In all the Iron Man comics, I can only remember Tony Starks being Iron Man. No son or anything as far as I know from Tony. This fourth film could create that character. If anyone could pull off being RDJ's son it's Jack Huston. Cody (or whatever you would like to call him) maybe locates his long lost father and they have some verbal and mental collisions which result in very entertaining scenes and at some point Cody has to take over for his old man.
Ed Harris as Iron Monger
Obadiah Stane becomes the President and CEO of his own company (Stane International) as a munitions dealer. He also goes into business with Howard Stark. After the elder Stark died in a car accident, Stane turns his sights on acquiring control of Stark International, the industrial corporation he had worked with, now owned by Tony Stark (Howard's son).