The upcoming game Callisto Protocol was at one point considered to be in the same universe as PUBG. However, some creative differences led to that aspect being cut out of the game altogether. Still, the fact that it was even considered showcases how much video games love crossovers.
Sure, there are the multiverse events like the Super Smash Bros franchise and Fortnite, but there are games that could potentially be set in the same universe as each other. Whether it's cute references or blatantly the exact same characters, these video games show that the MCU isn't the only one doing cameos.
Wolfenstein and Doom
Wolfenstein and Doom are the franchises that jumpstarted the FPS boom of the 90s and are incredibly influential videogames in their own right. What some people might not be aware of is that Doomguy and B.J. Blazkowics are actually related.
It's been confirmed by developers that B.J. Blazkowicz is the great-grandfather of Doomguy. Considering how over-the-top both franchises get, this is honestly not a surprise. The Blazkowics bloodline is a strong one, and they seem to really like violently purging evildoers.
Manhunt and Bully
One is a lighthearted and crass school comedy game while the other is a gritty, depressing murder fest. Of course, it makes total sense that they are in the same universe. Bully makes several references to Manhunt, but one, in particular, stands out as evidence that they are in the same universe.
Sometimes, NPCs will be talking about "seeing the new Starkweather movies" as the player passes them. Of course, Starkweather is the main antagonist of Manhunt. If Jimmy's character doesn't get any better, he's likely to end up like Manhunt's James one of these days.
Hitman and Kane & Lynch
Hitman, a highly rated series follows the exploits of Agent 47 as he is tasked with killing various criminals and political figures. It was only inevitable that he'd encounter some other mercenaries in the world. This is exactly what happens if a player visits a certain gun range.
Kane and Lynch from the game series of the same name can be found bickering and shooting guns in there. In a darkly comedic moment, the player could totally just kill them right then and there. It's pretty funny that an entire game franchise's story potentially ends with Agent 47 wordlessly killing them out of boredom.
Sleeping Dogs and Just Cause
Sleeping Dogs and Just Cause are both unorthodox sandbox games in the sea of GTA clones. Sleeping Dogs and Just Cause 2 take place primarily in Asia, and both focus on taking down huge criminal enterprises (kind of). There's also the rather blatant radio commercial.
In the commercial, the voiceover recommends visiting the beautiful islands of Panau, the same islands that Rico is in during the events of Just Cause 2. Not to mention, one of Wei Shen's outfits is Rico's outfit. Seeing as "beautiful islands ruled with an iron fist by a tyrant" is also a thing in Sleeping Dogs, there's likely an epidemic of those.
Battlefield: Bad Company and Mirror's Edge
Bad Company is a more lighthearted and comedic branch of the Battlefield franchise, but it's still pretty grounded and dirty. In contrast, Mirror's Edge is a dystopian free-running game with sleek buildings and minimalist aesthetics. Despite these differences, both games have strong evidence of being in the same universe.
For one thing, there is an audio log in Mirror's Edge that blatantly has the Bad Company crew doing their same old bickering schtick. Considering how much effort went into getting the same voice actors, it's doubtful it's just a cute reference. Not to mention, Faith's running shoes can be found in the Battlefield universe as well.
Ace Attorney and Dino Crisis
The two series couldn't be any more different. One is a pseudo-detective law attorney visual novel, while the other is a Resident Evil clone with dinosaurs instead of zombies. And yet, somehow, both are more than likely set in the same universe, since Ace Attorney features some of the same characters.
The biggest evidence is the main character, Regina, showing up in the game's canon posing as a car attendant, along with other Dino Crisis characters. Being a secret agent of the government, her being "out of character"could actually just be a cover, much like Black Widow in Iron Man 2. Not to mention, the country of Borginia is referenced in both Ace Attorney and Dino Crisis, further cementing the connection.
The Sims and Mass Effect
Considering how The Sims doesn't exactly have a "story" per se, it's odd to think that it could possibly be connected to Mass Effect, a sci-fi space opera of all things. Still, there's no evidence saying it couldn't be. After all, Mass Effect's Earth isn't all that weird.
In The Sims 3, the player is able to use a time machine and travel to the far future or far past. In the far future, there is a crowd celebrating how a "shepherd" saved humanity from an "ancient machine god". There's also the hilariously unfortunate implication that the player themselves might be a Reaper, controlling humans for their own whims.
Bioshock and Firewatch
Firewatch is a very atmospheric game that feels more like reading a snug mystery book than a traditional video game. Bioshock is also atmospheric, but crazy little girls are trying to stab the player. Still, the two share some connections that put them in the same universe.
In Firewatch, the player can find the book The Accidental Savior, a common sight in Bioshock. The developers have come forward saying that these light references are open to interpretation. There's also a fascinating theme at play here. Bioshock's story is the exact kind of excitement the Firewatch protagonist is chasing after. As it turns out, there is a grand conspiracy out there. The Firewatch MC just isn't part of it.
Watchdogs and Assassin's Creed
Ubisoft is fond of putting easter eggs in their video games, but it was only in Assassin's Creed: Origins that the tie was cemented. A previous game in the series had a Black Flag character Olivier Garneau as a target in Watch Dogs.
Further cementing this thread is Origins providing the player with CCTV footage of Garneau being shot by Aidan Pierce. Although Ubisoft does say these are "non-canon fanservice", that's only extending to main storylines. As far as gamers are concerned, the two games coexist in some strange multiverse, especially with Legion adding an actual Assassin in the form of Darcy.
Metal Gear Solid and Splinter Cell
Metal Gear Solid and Splinter Cell were the premiere stealth games of the 2000s. Despite this rivalry, both games have playfully thrown jabs at each other, and both are fans of each other's franchises. In a minigame on MGS3 called Snake vs Monkey, Snake asks if Campbell could just send "Sam or Gabe" to do the mission.
This refers to Sam Fisher and the now rather obscure fellow stealth operative, Gabriel Logan from Syphon Filter. In a sad twist, Sam Fisher also asks the same thing, wondering if "the guy with the bandana" can't handle this mission, only to be told he was retired (referencing MGS4's ending). He ends with "Then it's only me", a somber reminder that Sam Fisher's the last of his kind in triple-A games now.