There was a plan a while ago to open one in VA, outside DC. (My neck of the woods)
But it got killed for a number of reasons, one argument being that the Civil War historic battlefields in the area would have gotten trampled.
http://en.wikipedia.org/wiki/Disney's_America
Yeah, that was too bad. While NoVa isn't year-round resort weather, I'm sure Disney could have imagineered a resort that wasn't hindered by that. And of all companies, they would have respected whatever needed respecting. Now those same tracts will be covered with Applebys.
At that time I was relatively new in the work force and there's a good chance I would have applied to work there. I love my current job and am rewarded handsomely - but it's no Disney
I actually think the only answer to this question is something VERY close to
Disneyland. My reasoning: DisneyWorld draws from all over the US, except maybe a little less in the the SouthWest, which feeds DisneyLand. DL is, of course, out of significant expansion room. And it is all about getting more $ from each guest from each visit, which means getting them to commit to more days.
So imagine a large park(s) about 50 miles from DisneyLand, and a tract for hotels between the two sites (25 miles from each). Disney could begin to sell 4, 6 10 day packages to DisneyLand, and while not owning all the property between the parks, could do a reasonable job of keeping them on Disney soil, spending money.
So my vote is 40 or 50 miles from Anaheim.