What are some games that you guys wish would crossover

How do you mean "crossover"?

"Guess what I'm playing" and "Throw something at the User Above You"

I mean like what video games would you like to see collaborate

Personally, I prefer games standing on their own, in their own worlds, rather than colluding with others.
BTW, isn't Fallout 4 an example of mixing various worlds/ideas? I mean it certainly takes something from Minecraft, slaps some Fallout elements and also some Elder Scrolls, for good measure. Mudcrabs...err, mirelurks... Eh..
Reminds me of one of those songs with several artist on the title.

Ratchet & Clank 3: Up Your Arsenal should have had a level where they crash on the Wasteland from Jak 3 (outside Dr. Nefarious' sphere of influence) and need to get their ship repaired to resume their own adventure. (I know this is a little too much like that desert level in Going Commando, but bare with me.) Both games would share side-missions where each has you playing the title character(s) as usual but the other pair is with you. These missions would have the same banter, enemies, and objectives; each character would have their standard move-set as an NPC in the opposite game, but with additions based on the save-file of the game you're not playing. (Same as the way weapons would carry over from the previous R&C) To keep up with Jak in his dune buggies, Ratchet's spaceship can still hover, but not fly. When not palling around with Jak, Ratchet could get more missions from Cleaver by visiting the garage door, but he's not allowed into Spargus or Haven City. And in one of Dr. Nefarious' attacks, one of the victims in the background is an Ottsel that looks nothing like the ones we see in Jak 3; it would be the same kind of cameo as E.T.'s people in the Star Wars prequels. In fact, while we're at it, Angela can have a short scene in a linear Jak 3 level to jokingly explain her absence for R&C3 and onward. Finally, New Game+ (Hero Mode in J2/3, I know) would unlock the opposite pair as playable characters.

woah i like it

Silent Evil It's a Silent Hill game, starring Resident Evil characters and each one has their own story to play through. A tricky gimmick would be paying attention to body-language to determine between inner demons (any manner of melee or ranged attack) and zombies (lunge and bite); you can't survive without this knowledge because they're immune or weak to different weapons. Each character will see the Nightmare World differently and entirely different inner demons, but they all suffer the illusion of zombies being much stronger during nightmares. This illusion is in the form of exaggerated environmental interaction, like knocking objects around or tearing down doors, but damage to player health remains the same. To make what I said about doors possible, the game needs to be a sandbox; so doors are no longer loading screens, but they can be blockaded and broken like in RE4. (August 30 edit: I had a nightmare about this scenario, of inner demons with RE7/8 graphics breaking into a diner and the gross patrons act like nothing is wrong.) The save-file keeps track of each character on separate score-cards which you select to start or continue their story. If you assign the highest difficulty to this file, then your first character is at risk of hogging all the collectibles before others can get to them; so you need to balance having just enough health and ammo to get by so that your later characters also have a chance. To ensure the effectiveness of this restriction, there's a strict 1-file limit for saves, but that never stopped PC players from making workarounds.

I can't even begin to think which characters I want in this, or the set-up for their arrival.

Apart from the fact that i'm still waiting for the "Tekken X Street Fighter" game that was promised like 10 years ago, i would love to see a crossover between Final Fight and Streets of Rage, where you can play as characters from both series and fight against a collaboration effort between Belger and Mr. X to take control of the country.

Mortal Kombat and Killer Instinct.

Animal Crossing and Doom, obviously...

But which would be the baseline design? Is it Demons in a happy village, cute animals in a hell-scape, or some mixture like shooting cute demons with the weird Animal Crossing camera?

Now that MS own both Bethesda and Inxile I feel like a Wasteland and Fallout crossover might be in the cards. I don't mean officially combining the worlds or anything, but like having Rangers in Fallout or something.

+1 for a good idea

I always wanted a SimCity and FPS mix. First you build a huge city, then you walk around in your city and shoot stuff. And when it gets boring, you summon a natural disaster and watch the city burn.