I doubt that a major thrill coaster will replace Boomerang, its in quite a small area. The only possible thrill coaster I could see in its place would be a Gerstlauer Euro Fighter/Infinity Coaster or a Mack Big Dipper. I personally think a new flat ride will go in its place.
As mentioned above, there is a little more space than it initially seems, as a nearby flat and restaurant have gone. This is what they have to work with:
Plenty of room for something if they get creative. Space is so scarce at Knott's, I can't exactly see them removing a perfectly satisfactory coaster and flat ride to replace them with a single flat.
I can't see the Boomerang going to Cedar Point. Not only would location be an issue, it would just be so out of place and unpopular there. I would agree that it's most likely between Valleyfair and Michigan's Adventure.