There have been rumors, for awhile, that Marvel Studios is ditching the Phase aspect of their film mapping. Allegedly, the movies will be less interconnected, because of how difficult it was to create and maintain that detailed level of continuity for a decade.
With both Fantastic Four and X-Men in the fold, I highly doubt that Tony and Cap are recast any time within the next decade.
Who would the next 'big bad' even be? Galactus? Is he a bigger threat than a Titan who, with the Stones, could destroy the 616 universe..? I think they need to split things up not, and now plan towards a single focus, to keep things both fresh and scaled.