Re: Recast RDJ
While I don't doubt that there are a number of actors who could do fine jobs as Tony Stark, and while I understand that plenty of re-castings have been successful before, I believe that, if Marvel is smart, they will not re-cast, and will simply leave the character be for now.
Iron Man, as a character, isn't their moneymaker. Robert Downey Jr.'s Tony Stark is. Sure, you can do some really cool action with the suit(s), but there's nothing inherent to the character that audiences are in love with.
With regard to the Iron Man stand-alone series, I feel that 3 was a perfect closer—I wouldn't know where to go from here without undoing his arc or creating something that feels tacked on. And it's not like there are a wealth of Iron Man stories begging to be told or great Iron Man villains left to be shown onscreen. The series feels complete here.
With regard to The Avengers, I think they can start phasing him out. Make The Avengers 2 his final appearance. While Iron Man's appearance on the team made a huge impact in getting audiences to come see the film, it was, again, not Iron Man, but RDJ's Iron Man. I think most audiences would rather see the character sidelined than recast. If they're worried about losing that charismatic, cocky, smartass character with the great one-liners, then introduce a new character for audiences to fall in love with that can fill a similar role in the team dynamic. But re-casting would just be missing the point, and would feel wrong.