As far as a park, no. Even if there was a business reason for a park there (which there isn't), not enough room.
As far as a stand-alone Disney resort destination, a DVC variant has been the primary speculated potential use for that site. I rate the likelihood of that as highly questionable at best; Disney has yet to open any resort away from a Disney Park that isn't in a beach resort area -- and preferably a year round one. National Harbor meets neither criteria.
And as far as some sort of stand alone seasonal cruise ship pier facility, I'd also say no. Beyond no adequate space for a terminal and berth, the location doesn't make business sense: the market for home port sailings out of the "D.C. metroplex" is apparently limited, given only a few lines do Baltimore sailings, using smaller ships over a relatively short season.
The serious money in "home port" embarkations as you move up the East Coast is sailing from either the West Side or Brooklyn cruise terminals in NYC or out of "Port Liberty" (the vanity name given to the cruise terminal in
Bayonne, New Jersey). If Disney decides to give home porting a try I'd be
very surprised if they didn't notice the fact that RCI, HAL, Celebrity, Princess and NCL have all deteremined it's more than worth their while to serve that market with some of the biggest and best of their fleets.