It was said that the intitute is north of D.C. and "that one locatcion" must harbor the institute interesting thought

I believe that originally instead of Mothership Zeta there would have been an Institute DLC, however they've figured that either it deserves its own game or that they'll make it a DLC for Fallout 4. I have no evidence to support this theory however.

True but I have a feeling that it will be a location based around the game .....similar to the citadel in Fo3 and Hoover Dam in FONV