Post Potter is way too early for us to come to concrete conclusions, and probably for even Universal to think about. I think for a while, the only new additions besides Potter will be on the Studio Tour, where their construction and operation won't have an effect on the main park.
As Brover said, the park will need to have as many things open as possible when Potter opens. If they wanted another major attraction (besides Minion Mayhem) to be open by the time Potter opens, it would have to be something on the Studio Tour, or for it to replace something ASAP, like SES, which I don't see happening. The park can't afford to be a full construction zone.
But after Potter, the first thing I see being made into a new attraction would be SES or Shrek. But a good season or two after Potter opens and settles in.