hOld UP:
WHAT IF; Blue diamond found a human (taking Greg out of the question coz if we were going back all the way to the time when homeworld even found out about Earth in general and before Pink was given the colony, he wouldn't even exist, but lets say he is someone almost accurate like him in appearance-wise and personality-wise) on her visit to the earth before Pink took on the persona of being Rose Quartz.
She would take him back to Homeworld with him and then afterwards, introduces him to White, convince her (somehow) that he understands her and her emotions like no other (since she is appealed by the fact of how he is able to empathize and sympathize with her or others and his intelligence), and so, White allows her to have him. So they let him live on homeworld, but after finding out that he can grow old due to age, they use some sort of Homeworld technology to slow down his aging process tremendously.
After a while, Yellow Diamond finds out about this, and she also takes a liking to him, occasionally visiting him and Blue to have conversations about topics of all sorts, and finds him even more and more interesting to the point of developing feelings for him as Blue did.
Pink is to find out about this, and also pays him visits at Blues Castle (assuming they have a castle for each Diamond). She would also find him interesting in the way he does things, and soon becomes fond of him, and she too also falls in love with him as the other two Diamonds.
White Diamond also falls in love with him for some reason (maybe she likes the fact that he thinks himself as an individual, instead of some part of the Human race and expendable, much like how the Diamonds are unique and are like no other) and falls head over heels to for him, much like all the other diamonds.
The Diamonds begin to smother him in many ways, giving him all the attention he could ever desire (much like how the Diamonds smother Steven with attention in Steven Universe the movie.) In some point in time, let's say that he teaches them about sexual intercourse or reproduction (the human way, not how gems are made) and so, they, through some messed up Homeworld thinking, they would take their place as leaders of Homeworld, and continue the Diamond Hierarchy.
So, skipping the process of how he does the sex-thing with them, the diamonds have children, all boys (probably just a different version of our steven, with different personalities), and the Diamonds disappear as a result. Let's say the Diamonds assigned the most elite gems about it and informed them about how they should raise them so that they could continue the Diamond rule over Homeworld.
Let's say that they are raised in Homeworld style, but because of their human father, who thousands of years ago, while being under the Diamonds care, he observed how shitty and messed up society is, decides to teach them his own style of thinking, all without the elite gems knowing since he spends most time with them, and after the four boys reach the age of 9 or 10. He makes them understand that he was kept here without his consent, even though he actually loved the diamonds as the mothers of his kids, he still wasn't ok with the fact that his sons were going to grow up in this society in the future, and so, after telling them stories about how terrible Homeworld is, and through the wonderful stories he tells them in the boys' childhood about his home earth (which halted all efforts to be colonized by the diamonds under the father's plea along with Pink Diamond, something which they agreed upon only by the fact that they loved him, and would have just laughed it off if they didn't, something he was thrown completely off by) and how it is best that they left, they make it a promise to each other one day escape Homeworld back to their father's home on Earth.
He teaches his sons about the good qualities (like the genuinely good qualities) of the Diamonds and even taught them that they were good people, and also taught them that society isn't what it actually seems. Through him, the four sons find out about the homeworld's strict-ass rules and how they basically condemned all types of self, and how all types of gems should contribute to making the late Diamond Empire perfect.
During this whole time period, it is also worthy to note that few 'chosen' gems of the homeworld started to experiment on fusion, and the gems closest to a diamond and most Elites upon Elite gems under each Diamond (four in existence, acting like the Diamonds' second in command), White Moissanite, Yellow Moissanite, Blue Moissanite, and Pink Moissanite go ahead and try to fuse, resulting to a Paragon Diamond gem, and since nothing like it has ever been seen, and since a Paragon Diamond is the most flawless most illusionless Diamond in existence, makes themselves as the new rulers of Homeworld, and still carry out the rules in homeworld, and due to paragon being perfect, oversees all that happens.
This puts the original heirs of Homeworld in danger of being shattered (or something horrid), but after a private meeting with Paragon, make an agreement to let the children live untouched by homeworld, at the cost that they leave and seek refuge on earth, previously abandoned by the Diamonds due to Pink Diamond and the dad's plea to save it, and if they ever even dared to come back, they'd execute them and the planet.
I haven't planned the rest out at this point, so I'll leave the rest to you guys to maybe try and figure out how the story unfolds.
