I honestly don't think the outcomes of Tomb Raider and Son of Beast would have changed much. Tomb Raider was frankly unsustainable on a seasonal park budget, CBS was apathetic, and park attendance and guest spending slumped during the 2008 recession. If CBS had owned the parks up to the recession, I think Tomb Raider might have gone away even sooner than it did. I'm assuming that Son of Beast's 2006 accident and 2009 medical incident would still have happened in our hypothetical world; if so, I don't really believe that anyone in charge would have tried to redeem it much more than Cedar Fair did. Maybe some sort of leadership change at Paramount Parks during the rise of RMC might have saved it, but if PP's leadership around 2005/2006 was any indication, I don't think they would have been in the business of carefully trying to untangle the liabilities of that ride instead of just replacing it.
As far as additions go... I imagine it would have been the "cheap and cheerful" route through at least 2008. If a new coaster showed up, I would imagine it wouldn't have arrived any sooner than 2009, and even then it would be because it was already well in the works prior to the recession. CBS wasn't enthusiastic about owning the parks; they wanted to get rid of them. If we're not selling the parks in this situation, then I imagine that some amount of restructuring within Paramount would have put the parks in some other Paramount-owned brand's hands sooner or later. At that point, maybe we might have seen some more movie-themed E-ticket rides arrive at the park. Oh, and maybe Flight of Fear would have gotten an MTV-related makeover that was rumored at one point. Kings Dominion either wouldn't have a Flight of Fear anymore, or it would be a completely different experience.
Winterfest-wise... I dunno if it would have actually come back in 2006 or not. If it did, surely it would have had more attractive pricing than the 2005 version.
I don't know what else you could predict more than a couple years beyond 2006, but if we're open to daydreaming, it would have been really interesting to see them switch hands after the recession and focus on turning the park into some kind of year-round destination with Universal- or Disney-quality rides. I realize that's not what Kings Island was originally meant to be and not what it is now, but that's where early-2000's KI was trying to head, in my opinion. It didn't work, but as an eternal fanboy of Tomb Raider, it would have been cool to see that level of detail work out and inspire similar additions.
I know the original post specifies that the Paramount parks weren't sold in this situation, but I honestly feel like the parks would have been individually sold one way or another if Cedar Fair hadn't bought the lot. I don't know who would have bought each of them, but it feels like KI would have gone to Cedar Fair sooner or later. There was a lot of heavy speculation and rumoring around then that Kinzel wanted Paramount Parks specifically for KI. From a business perspective, that makes some sense.
This was a fun topic to think about. Thanks!