I don't think they should build a themepark devoted to thrillrides entirley, but im not against them sprinkling them throught thier existing parks.
But, when you think about it, how do you theme a giant rollercoaster?
Basically all of disney's big rollercoasters are themed to mountains, but a mountain can only be so big. They would have to get pretty creative to get it done, not to mention spend a buttload of money.
I'd like to see them try, but im fine with them just sticking to what they do best.