That already exists. Not as a game though but as a series. It's called "Happy Tree Friends".

Here is one episode of it: https://www.youtube.com/watch?v=opAinuVvKfw

WARNING! That link is totally NOT safe for work. Don't click unless you can handle extreme levels of violence and gore. Don't blame me if you feel sick after watching.

The King of Fighters vs Skullgirls

Danganronpa with any other game that takes place in a high school, especially Hatoful Boyfriend.

I've known of them for around a decade, but I never bothered watching because slapstick cartoons aren't much for me even without extreme gore. Besides, gore alone doesn't invoke Doom-adjacency to me; if some of the HTF were hellish abominations, maybe.

Motorstorm: Refueled is a sequel to Motorstorm: Apocalypse and prequel to Fuel. The cataclysmic earthquakes have settled down, but the thrill-junkie racing culture that they empowered will never die. Much of Continental United States is evacuated, leaving what remains of the land to the only remaining users of fossil-fuels. The game would feature the entire states of Nebraska, Kansas, Oklahoma, and Northern Texas in a single map (as opposed to Northern California and Western Nevada in the original Fuel). The theme of the landscape design is the scars of heavy natural disasters being fresh and comparable to the Boston bomb-site in The Last of Us; perilous terrain is adorned with stunt-structures to make the most of the danger. All racing game-play is taken from Motorstorm because Fuel was sadly weak in that department, but there are vehicles from both titles. To establish why there's no boosting in Fuel, during every race, there's a permanent countdown in the corner of the screen indicating how much boost-time you have until it runs out forever, and the top 10 or so hardest races are impossible without boosting. You can regain boost-time by re-doing races with better performance; any race where you get 1st Place without boosting at all is marked with a diamond trophy. Starting-Points are marked on the map and each race begins with your custom character in motion-comic style speaking with named NPCs.

Empire Earth and Age of Empires, obviously.

Bad joke aside, a weaker example would be Project X Zone 3 is something that really should happen but probably never will. I would love it if Bayonetta actually got to be in it as well as: Ayane, Kasumi, Ryu (Ninja Gaiden); Samus Aran (Metroid); William Adams (Nioh); 2B (Nier Automata); and more but I cannot remember them right now, I might have to add to this post later.

I was recently contemplating mixing elements of Dead Space to the Metroid setting. Up the horror vibes, even if it uses green blood and blowing sections of alien critters to bits instead of dismembering it could make for an interesting re-interpretation of the Metroid setting to add back some of the creepy elements as well as the feeling of isolation one feels playing the first Dead Space title and the first Metroid. Also, honestly, the first time I was attacked by a Metroid in a demo of Metroid prime hunters it made me jump and ditto when this happened the first time in Prime 2. The Metroids are already pretty creepy, it would not be hard to see them as horror game antagonists, especially when you consider they are supposed to be tougher and more dangerous than the average creature anyway encountering them could resemble the tougher enemies in Dead Space or a Resident Evil title. Having the game use a third-person camera or first person camera would be a tough call. Dead Space gameplay would be great as Dead Space is just an excellent way to do action but it could lighten the horror element. It depends if the closer perspective to the creatures (like I experienced with the Metroids) or seeing the repercussions of falling victim to these creatures is more frightening. I dunno. I don't actually play a lot of horror games and I would think this theoretical game would benefit from being an action/adventure/horror balance would be best in the same vein as Dead Space started off with the first game.

The Chicken Run game.

hmmmm himeko sutori and baldurs gate is one ....

himeko sutori and star wolves might be another or ....

total war, supreme commander and galvic.. or maybe better yet, stellaris ...

or himeko sutori and final fantasy ( though last remnant might be what comes out of the equasion )

himeko sutori and tetris

himeko sutori and doom .....

I think Portal and Assassin's Creed could crossover. Aperture is part of Abstergo, obviously. And then you could use portals for assassination gameplay as well.

Since you're going for combining the stories, it's worth noting that neither of them have the kind of time-travel that allows bringing future technology into the past, unless you want to say G-Man displaced Chell into the past for whatever reason, and I guess murdering corrupt people in power is as good a career move for her as any.

But I'd much rather the combo be legally distinct from either property because every iota of the future that every single Asscreed game references ruins an otherwise fine experience. It's bad enough it all started with some tosser using a "what if" to justify bullshit sci-fi, but it's been a consistent rule every single time that if we were just in the various time-periods, the story would be 1,000x better. Just forget anything after the one in England.

How about inFamously Uncharted, a DLC about Nathan Drake (and at least 1 friend) entering Empire City or New Marais to nab himself a Ray-Sphere only to discover it's more trouble than its worth. Every mission he goes through involves parkouring over the same buildings that were already there, but they're enhanced with Uncharted's trademark illusion of peril so that climbing surfaces crumble just as you pass them.

Every game of a property with Lego ties should have Lego-skin mods or DLC. It'd be more easily done and less redundant than making Lego game remakes of Batman or Star Wars games. I'm looking at you, Portal and Sonic in Lego Dimensions.

The Witcher and the Dovahkiin together, in a fighting game.

Witcher gameplay in (early) Doom levels would be all kinds of lame, like bringing ornate dueling swords to a gun-nut's hoot-a-nanny and everybody's too drunk to see above 8-bit. You must be talking about the other way around. Freely switching between blade-dancing and giant guns to really give ugly critters of both games what-for does sound like a blast, even if it's in the relatively bland hellscapes of modern Doom. (Emphasis on relative, of course.) Actually, Oblivion-style gates scattered around the Witcher 3 map to give access to Doom levels quite completes the idea.