It doesn't have a bunch of coasters, but from what I've heard, it's one of the greatest parks on Earth.
It looks like you have the eastern US covered! Maybe trade Six Flags America for Six Flags Over Georgia?
Here's what I'm kinda thinking.
Six Flags Great America
Six Flags Over Georgia
Busch Gardens Williamsburg
Six Flags Great Adventure
Canada's Wonderland (If you can)
I would recommend 2 days at Cedar Point.