Hmm...this depends. CBS ownership, or does Viacom get to keep the parks after the 2004 split? CBS did not seem to care about or want the parks, so if Cedar Fair had never bought the parks from them, I honestly wonder if Kings Island would even exist in 2018. As far as I know, CBS might have even sold the park so it could be razed and condos built on the site, which would obviously make tons of people angry. Thankfully, IRL, Cedar Fair did end up buying the parks from CBS.
Now, if Viacom had acquired the parks in the CBS-Viacom split, maybe we have a different story. Viacom has made a few attempts to build new parks since losing the old ones (though so far, none have stuck) so I think they would have actually wanted to keep and run the Paramount parks. So, under Viacom...
Son of Beast: Still gets torn down. This ride had many major issues, and RMC did not really get going until 2012-2013, after Cedar Fair had destroyed SOB already. I think Viacom would have been even less tolerant of the ride's major flaws, and it may not have even lasted until 2009 had they stayed in power.
Flight of Fear: Probably re-Themed to a Musical Roller Coaster, but remains in the park. (There were actual plans for this, which Cedar Fair reversed) The Kings Dominion model may have been moved to a new park entirely. I'd say...Carowinds.
Banshee: Not likely to have been built. In this scenario, Cedar Point is still a "enemy" rival park run by a different company. When Cedar Point built Raptor, they had B&M make a deal with them not to build any more inverted coasters within 200 miles or so of Cedar Point. Kings Island is within this range. I don't know if that deal had a expiration date or not. If not...uhh, anyone want Kings Island to get a Vekoma SLC?
Diamondback: Unlikely- after SOB Paramount & Viacom seemed iffy on huge roller coasters, but since Cedar Point never added a B&M Hyper, still possible. Probably would not have been built in the current spot, however, since Paramount was not as big on tossing rides "In Your Face" like Cedar Fair does. Might have actually been the SOB replacement, TBH. Would not be called Diamondback though, of course, if Viacom/Paramount built a B&M Hyper.
Firehawk: Might still be X-Flight @ Geauga Lake, but could have been re-located, maybe to Dorney Park or something. However in this scenario I think GL has a much better chance of surviving, but Cedar Fair did have plans to return it to its family park roots and downsize it before the PP Acquisition drove them into massive debt making that impossible.
Tomb Raider- The Ride: TBH, unlike SOB, TR:TR has a bit better chance of still being around in this scenario. Running it in crazy mode in 2007 took a LOT of years off it, so had that never happened, maybe it would have lasted longer. It would probably have been pretty run down by now though, and likely still be a shell of the ride it was in 2002, unless at some point Viacom put money into refurbishing the ride. It may have gotten a re-theme though, since the Tomb Raider movie is very outdated by now.
Mystic Timbers: ...might actually still be built, but under a different name. GCI's are not expensive, or overly large, and the company has a great reputation so Viacom probably would have taken interest and wanted one. With Son of Beast still flopping, the park would end up needing a modern woodie again anyway. In fact, under Viacom, I think smaller coaster companies would have some rides at Kings Island by now, like Gerstlauer, Mack and Premier Rides since Cedar Fair would have most of the big B&M's on lockdown...unless KI built them first, should Viacom have opened up to big coasters around the 2010's. A Dive Coaster seems like something they could have done, and ironically it would prevent Cedar Point from getting Valravn!
Other Notes: WindSeeker probably does not get built, but Viacom might have built a Star Flyer of some sort instead. Racer still runs backwards since apparently that was a Cedar Fair decision, if Viacom is not tempted to RMC it. TOP GUN, Drop Zone, FACE/OFF, and Italian Job: Stunt Track retain their old themes and names. Nick U remains the kids area, but would need some re-theming as lots of the shows the rides were based on are over now. Boo Blasters is likely still Scooby Doo. Crocodile Dundee's Boomerang Bay probably remains the name of the waterpark as well, though it would also have probably been upgraded and gotten new slides by now. Unsure if Viacom would allow Vortex to live as long as Cedar Fair has, since the policies on very old rides like that could be different (Six Flags has destroyed most of their Arrow Loopers, for example, while Cedar Fair has kept most of theirs so far).
Well, those are my predictions for this scenario, for at least the major attractions and stuff. Though since Cedar Fair bought the park in 2006 IRL, none of this is true in this universe, and some of it may be a bit optimistic at that.