Take four of your favourite OCs, from different universes. Characters who cannot meet, characters who have no shared space in which to meet. Characters who are connected to their universes.
Now, pare those characters down as fine as possible. Try to make them fit in a universe other than their own. Slice away every single element of the character that can be considered less important to them. Does it really make a difference that they use a sword instead of a gun? Why? Why not?
Continue with this process for all four characters. Now that you have their barest elements, when you know what those characters ‘need’ to exist, in their settings. Then start reconstructing. Try to craft a universe around the characters you have. This character needs a war to have been injured in. This character needs a form of long-distance travel. This character needs a hegemony to rail against.
See what it takes to make these pieces come together anew.