A recap on the top three possibilities from what we know:
#1 Helix-esque coaster with possibly the most inversions or fastest speed. A first being the steepest vertical drop on a Mack Mega Launch Coaster. Expect incredible G-forces and/or airtime.
#2 As mentioned before a while back, this could still possibly be the first true Mack Dive Coaster using their new Big Dipper cars. While that alone being a first, it may also include the fact that it would be the longest Dive Coaster ever built and the first Dive Coaster with launch elements. (I do not count Lost Gravity as one as it is more of a Eurofighter in terms of its layout.)
#3 The widely rumored Extreme Spinning Coaster. Same layout as the Mega Launch Coaster but with a multi-car coaster train that either has all cars spinning or some spinning. Several launch elements/sections will be included along with possible elements/sections that will do a controlled rotation thru segments of the ride like Cobras Curse. Firsts will of course be the first spinning coaster with a vertical drop and the first with inversions. (If you do not count Blue Fire with the spinning car on the rear)