Shipping Markiplier's guy from Iron Lung with Ryan Gosling's guy from Project Hail Mary is a great example of a crossover ship that is not, per se, a crack ship.
These two never met and the stories they are from are not technically all that compatible (Grace is trying to stop the stars from going out, Simon exists in a far-future society where they already did), and yet, the themes and circumstances of the characters do create the irrefutable impression that if they ever DID meet then they would have a whole lot of feelings about it. They would not, indeed could not be normal about each other without that seeming weirder than if they had sex about it.
