Looking years (a decade?) into the future, it would be prudent to go down Nolan's Bruce Wayne/John 'Robin' Blake route.
After Downey's ready to ride off into the sunset due to age and/or ennui, it would be really cool if we see an Iron-Man-in-Training type young 'un whom Stark would mentor and who would take on the mantle of Iron Man.
While expanding Nolan's Batverse into the Man of Steel, Justice League and beyond CLEARLY wouldn't work, the MCU's a completely different monster. The MCU was constructed as a shared universe since inception (not to be confused with I
) and the movies, to a greater or lesser extent were meant to be linked, thematically.
This would cleanly side-step the need for a 'universe reboot' (an abhorrent idea and my own personal worst case scenario) and would allow the Avergers franchise to move forward with their flagship character in place.
The key, of course, would be casting someone who holds a torch to Downey in the charisma department.