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 combines My Little Pony: Friendship Is Magic with Doctor Who, two franchises with very different atmospheres, in a single piece of artwork.note
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, like finding out how well waffles, pickles, and lemons go together.