I personally want to see them "pass the MCU torch" to Cap or Thor as the new star of the MCU. Probably Cap more than Thor just since he's more Earth oriented and basically the leader anyways. Tony started the MCU off with a bang, now let's give Cap and Evans the spotlight as the big dog in the MCU.
I see Tony coming back in Avengers 2 for sure, and I would assume 3 as well but maybe only in a Director of SHIELD type role in 3.. Iron Man has basically had his story arc play out in his 4 appearances and I personally loved the journey.. there's definitely more stories to be told, but I think a recasted, but NOT rebooted Iron Man 4, in Phase 4 is where you do it.
Maybe RDJ can play Howard Stark once he hangs up the Tony persona lol