The Men in Black (I think we’ll have to use the cast of MIB International, I doubt we can get Will Smith and Tommy Lee Jones back for this) have noticed a dramatic influx of refugees fleeing some great calamity. While trying to figure out what, exactly, is going on, one of the refugees starts twitching wildly and ranting about the coming of their god who will “restore the natural order” and suddenly their skin starts rippling as a symbiote forms around them. When their natural symbiote suppression system kicks in, it easily evades them because it’s not actually a normal symbiote: it’s the ghost of one. Loud noises still manage to scare it off but (after a humorous scene where nobody can actually hear each other because of the loud noises temporarily deafening them) they have to go to the experts on trapping ghosts: the Ghostbusters (I think we can get the surviving originals back but the 2016 versions would work too and allow for scenes where they can’t keep Agent H and Kevin separate)
So they go to meet the Ghostbusters and here we have an infodump on Knull, or some version of it. At the end, they neuralyze the Ghostbusters thinking they have all the information they need, but after they leave we find out that one of them (Holtzman in 2016, Ray in original) is aware of their techniques and has set up a post hypnotic suggestion in all of them that prevents neuralization. While the Men in Black go after Venom (who they’ll have interacted with offscreen between movies) for his assistance the Ghostbusters will go after the only human whom Knull fears: Mr Miyagi
Do note that all of this time, in the background, we’ll have scenes of the ghost symbiote infecting people and raising a small army comprised mainly of whatever actors we can pull from the component movies. Maybe Carnage comes back as a ghost, IDK
If it’s possible, I’d love to get Hilary Swank back just because Daniel-San is already in Cobra Kai so uniting both of them will be a big deal when just having him show up again wouldn’t. Turns out before he passed away Mr Miyagi taught them each half of a technique that would utterly destroy symbiotes, leaving no soul behind. Or maybe it’s just somehow effective on alien ghosts so that they can be trapped by normal ghost traps, we’ll play it by ear. Cue training montage where they just barely figure out how to combine the two halves of their teaching into the one special ki technique needed
So the MIB, having gotten Lisbeth Salander (I know her inclusion makes no sense, but I have even less scruples about that than Sony does), one of the tech-savvy Charlie's Angels, and Q from James Bond to integrate their own technology with that of the Ghostbusters and with some help from Venom and maybe James Bond(?) start fighting the ghost symbiote army but it turns out they can’t get the original without the technique. The Ghostbusters and the Karate Kids come in at the last minute and take it down with their super-powered crane kicks and the Ghostbusters trap it, but not before it leaves some cryptic message about how Knull is still coming and you can’t crane kick a god
Final scene before the credits: Daniel-San and Eddie Brock are sitting at a table roughly summarizing the plot to a third party saying that they need his help to find some artifact, with regular interruptions by Venom saying “Eddie, ask him!” over and over and over before finally Eddie yells “alright! I’ll ask him!” Before saying “okay, before we go on I have to know: have we met before? Because you look familiar,” at which point the camera shows us Tom Holland as Nathan Drake saying “I’ve never seen you before in my life, now where is this jungle?”
Mid-credits scene: Daniel-San, Eddie Brock, and Nathan Drake (plus whoever else we want in the next movie) are on a plane when Nathan says “I thought we were going to some exotic jungle, why are we flying to New Hampshire?” And then we hear the drums from Jumanji beating
The sequel revolves around the Karate Kids and Nathan Drake trying to infiltrate Jumanji in order to find some way to get the jewel out. Q is working with Spencer from Jumanji outside of the game while in the game Daniel is Dr Bravestone, Julie is Ruby Roundhouse, Eddie Brock is Shelly Oberon, Venom is Mouse Finbar, and Nathan Drake is a CGI Nathan Drake that looks and sounds exactly like the one from the games and constantly says things about how he doesn't belong in a video game or whatever. Meanwhile in the real world the game keeps spitting out jungle threats as it did in the stinger of the second movie. I guess the kids from the film franchise are dealing with them somehow, meanwhile Kate McKinnon keeps showing up at seemingly random places and being helpful
About midway through the movie we get a few twists: the game glitches and everybody in the game becomes their real world selves while the kids in the real world become their avatars again; Rhys Darby's character starts showing up more, threateningly speaking for the game in what appears to be clipped together statements from other cinematics that could theoretically exist in the game, and Kate McKinnon confronts Q saying "It looks like you need the multiverse's leading expert in transdimensional theory. Lucky for you, that's me. Jillian Holtzmann, nice to meet you."
I'm thinking that would end with them finding some way to trigger a video-gamy-style glitch that duplicates the jewel, leading to Rhys Darby as Nigel becoming some giant monster made up of jungle parts with the area that the jewel needs to be returned to being his eye. Nathan Drake climbs up top and manages to do it, and "Jumanji" is screamed out right as Nigel is smashing them with his hand, sending them back to the real world as per shaking his hand in the original movies. Not sure where to go from there just yet but I figured a good "oh ****" moment for a midcredits sequence would be the Jumanji kids, in their normal, real world forms, suddenly discover that they have the three line "extra lives" tattoos on their arms. Spooky but I have no real idea what that should mean