Grayling, Michigan is right near the heart of the ski area in Michigan. If you think the tourism ends when winter comes, you've never been to Boyne Mountain once the weather gets cold. Add to that the snowmobiers and cross country skiers, and there are a lot of people heading north in Michigan.
Wisconsin Dells is also in a very cold climate, and, though I've never been there, they seem to be doing fine. They also have one of the largest indoor water parks.
An amusement/theme park in this area would be a huge kick for the local economy. There are few large cities in this area, and that many jobs would make a big difference.
BUT...since I live in Southwestern Michigan, close to the Indiana border, I really wish they'd build something down here! It's several hours to any parks from here, and with the Indiana Toll Road running from east to west, the draw from Notre Dame University (an hour south from where I live) and the zillions of hotels that have been built for the ND football seasonal visitors, this would be a prime are for a park.
I hope this isn't just a rumor. Michigan really needs a boost to it's economy and more jobs.
And big coasters!