Rule 50
Internet Rule 50: Every franchise will eventually have a Crossover with every other franchise in some form. In other words, if it hasn't happened yet, someone's probably working on it now.
This trope includes any type of contact between franchises, whether it be physical or merely conceptual. See the image, which incorporates characters from My Little Pony: Friendship Is Magic and the graffiti meme from Silent Hill in a single piece of artwork. (Friendship is Magic, in fact, may be the single most infamous example of this trope in the early 2010s, having been crossed with everything from Super Mario Bros. to Pokémon to King of the Hill to The Amazing World of Gumball to even Fallout. Ranma ½ is an earlier claimant to the "most often crossed-over" crown.) Another example is Sailor Moon meeting World of Warcraft, two franchises with very different atmospheres.
The resulting crossover may be art or fiction. It may be a delightful surprise, like finding out how well chocolate and peanut butter go together, or it may turn your stomach.
This can apply to a Common Crossover, Mega Crossover, or even a Massive Multiplayer Crossover but not a Crisis Crossover which features characters from the same universe.
Note that Rule 34 refers to prurient versions of single franchises, while Crossover Ship refers to romantic crossovers only.
Rule ⑨, however, has has its own page.