Hmm...reading this, I thought of the California's Great America proposal thing I saw, where one of the pictures was a Intamin Impulse coaster similar to Wicked Twister- made me think the "returning" ride manufacturer could be Intamin, with one of those next year, which if Intamin and Cedar Fair were to work together again would probably be the type of ride Cedar Fair would get from them- it's a old proven model (they know how it's supposed to work by now), and isn't crazy expensive (though using Wicked Twister's price tag and inflation calculations it would cost around $13 million but that'd still be far less risk than spending $20M+ on a Banshee or Fury 325 sized ride) so if Intamin screws it up, it does hurt Cedar Fair as much as a more expensive ride failing would.
Though another possibility would be Mack Rides, which makes sense as I could see Cedar Fair wanting to work with them as they have had success with their rides in Europe and Asia, and don't seem to have any major reliability issues. Plus Cedar Fair has worked with Mack before, I know they built Knott's Wild Mouse Coaster in 2013. (RCDB Link: https://rcdb.com/11074.htm)
Though the image in that proposal was still a Intamin Impulse, but... #1- That could have been a placeholder image and Cedar Fair really isn't adding a Impulse to CGA or #2...Mack has their own model of Impulse coaster now, and Cedar Fair just used the Intamin as a placeholder for the image since the Mack version wouldn't have been built yet. This could also be the "never before seen coaster", as it would be a brand-new model from Mack, though Intamin did make it first.
If the "mystery coaster" isn't a Mack or going to GCA, then I am a bit more stumped on what else it could be though- RMC T-Rex perhaps?
At least we know full well what the 2 "Wooden Coaster Transformations" are though- RMC Iron Horse conversions on Mean Streak & Hurler. This makes it seem like come 2019 and later, Cedar Fair is gonna be building coasters from a lot more than just the usual B&M and GCI options- though I would not be shocked to see more from them as well in the Cedar Fair chain in the years to come.