Well, this document is very interesting. I would say I am about 90% sure we are getting a B&M in 2020. Is it the long-rumored Giga? Possibly. Could also be...
Dive Coaster: These things are GP magnets, Valravn at Cedar Point is crazy popular (it drew a ton of people away from Millennium Force when it re-opened on a Friday when I was there, Millie's wait dropped from 90 minutes to 45 almost as soon as Valravn when up) and they are not as expensive as other B&M models, so I would not be 100% shocked if Kings Island got one, though they did just announce Yukon Striker for Canada's Wonderland and Valravn is in the same state and is still pretty new, and they may not wanna undercut Valravn's long term legs.
Floorless Coaster: This is another option, though they would need to make it huge and do stuff Banshee does not do to keep the 2 from feeling too similar. However, it would not be much cheaper than the Giga, and since so many people seem to want a Giga, why not just go for that?
Wing Coaster: The craze for these seems to have died down slightly in recent years, but this is still an option. They could also pull a Thunderbird on this (TBH Floorless too) and have it be launched, though Kings Island is pretty close to Holiday World and I imagine people would notice if KI ripped off Thunderbird. Also, this would also be fairly expensive and not too much cheaper than a Giga.
B&M's other models are options, save for a Hyper and Invert since KI already has them, but the chances for any of those seem VERY low to me. There is also the option that KI gets a brand-new model of B&M, such as the rumored 4D coaster, but we would have nothing to speculate on that until construction started.
If I was gonna bet, though, I'd go Giga. There have been enough requests for it, it was speculated for BOTH Banshee and Mystic Timbers by fans (some of whom got annoyed when it turned out those were a Invert and GCI Woodie) and it would be a HUGE draw to Kings Island. Millennium Force no longer needs protected, it will be 20 years old in 2020 (off-topic: I see MF becoming a ACE Landmark in 2020) and CP has new draws like Valravn, Steel Vengeance, and possibly their own new 2020 coaster. If KI and CP both get huge new coasters in 2020, it could draw people to BOTH parks, meaning even more money for Cedar Fair. It would also be a warning to Kentucky Kingdom not to step on KI's toes, since there are rumors about KK getting a hyper coaster. It just seems to make sense- though if we do get a Giga in 2020, I would not expect another big coaster for at least 4-5 years after, so not sure what the park would do for its 50th in 2022. Probably bring back Phantom Theater or something?