They would have to pay very top dollar for any adjacent property. And what could they build on current site of of the Harbor Motels? There really isn't all that much room there. Definitely not a theme park. Maybe hotels, but will the people who currently stay at the relatively cheap Harbor motels be willing to pay more (say 2X) to stay at a new Disney hotel complex, or will they just stay at cheaper digs a block or two south?
I know that some have speculated that Disney could build an equivalent of WDW's All Star resorts there, but the land would be so expensive that I'm not sure they could charge enough to cover it.