Universal usually announces an attractions replacement pretty quickly but any details can take a while. Here's a timeline for Diagon:
December 2, 2011: Universal announce Jaws closure
December 6, 2011: Universal announce a HP expansion is coming
January 2, 2012: Jaws closes
January 25, 2012: Universal announce HP will replace Jaws and that the HE will connect the two lands
May 8, 2013: Universal announce that the expansion will be based on Diagon Alley and should open in 2014
January 23, 2014: Universal announce the name of the main attraction (Gringotts)
June 18, 2014: Diagon media peviews
July 1, 2014: HE soft opens
July 3, 2014: Diagon soft opens
July 8, 2014: Diagon officially opens
You can take from this what you want but I can see Universal aiming for a summer 2019 opening date if they turbo on.