Never gonna happen - Disney will not build an "all-thrill" park...their prime demographic is families w/ kids, moms, dads, grandma...they were worried when Universal opened, the Uni peaked and waned, opened IOA, peaked and waned, now with Harry Potter it will be the same thing. (Harry Potter is not a theme park - just an "island" in IOA, it may be cool, but it's two/three rides and some shops - people will go once and most will not rush to return.)
Anyways, a theme park with all thrill rides would have a limited audience, parents with little kids won't want to go there, Grandma won't want to go there. Forget it.
I think Jim Hill's idea is more likely - a "high-end" park like Discovery Cove with limited admittance but high prices. There are tons of people going to Disney with a lot of money - explain why there are 12 deluxe hotels and only 4 each value and moderate. If it means keeping the other parks somewhat reasonable in cost, than I am all for it.
Though I personally think Disney has no interest in additional parks - the "Master Plan" was to build enough to keep people there for a week without leaving - they did it and it worked. More parks will just feed off itself unless they have so many people coming to WDW that the existing parks can't handle them all, and they are not there yet.
SkierPete