AttractionSpot posted this video of the final ride on Boomerang, and in the description, he says that it is rumored to be replaced by a Gerstlauer ride, and that it will be a high thrill coaster. Sounds like it might be something like a Eurofighter or an Infinity coaster, which would be a good fit for Knott's in my opinion.
He also said that Boomerang will be scrapped and will not be relocated, which is good news if your home park is Valleyfair, Dorney Park, or Michigan's Adventure.
[youtube]https://www.youtube.com/watch?v=DebfLw-gMi4[/youtube]