So Fusion has usually been used as a metaphor for many types of relationships: Marriage, Abusive, Friendship, Siblings etc. However one I don't believe has been considered yet, is an extreme co-dependent relationship. Similar to Jasper and Lapis, where it would be unhealthy, but in a different aspect.
To put it into the context of the show, it would be in the form of a 5th Diamond and her pearl. So before the current events, the Diamond would have become too reliant on her pearl for her 'smarts' and to make decisions. At the same time, the Pearl, due to her gem type, crazed power, something she could never get herself, only gaining it through manipulating others like her Diamond.
Eventually, they become so dependent on each other, they decided to fuse which forced the other Diamonds to outcast them, Pink Diamond becoming her replacement. They were fused for so long, on the rare occasions they unfuse, they lose all sense of invisibility, with none of their old unique personality traits anywhere to be found. Apart they can barely function, only able to obsessively seeking each other out, to the point of a mental breakdown.
However, things didn't end here. When they discovered how 'wonderful' fusion was, they wanted as many gems to experience it as possible. Becoming obsessed with fusing. Wanting as many gems to fuse as possible, until it becomes their outrageous goal for the entire Gem species to fuse. The only reason stopping the unstable fusion from collapsing, is the Diamond's power. Proven by Lapis, a fusion can be forced together by raw power/steer will alone.
The spaceships of the Homeworld Diamonds, those of: Arms, Legs and Head, would be specifically designed to take on the growing monstrous fusion, slowly making it way back to Homeworld from deep space, only a matter of time before it returns.