I may be a bit rusty when it comes to the details, but one thing that's always disappointed me about the setting of Steven Universe is how much humanity and their relationship with the gems gets shafted and pushed to the wayside. For an alien civilization that had been on Earth with humanity for nearly a thousand years (according to the timeline) that scarred the lands in a war of heavenly proportions and left behind untold numbers of supernatural structures, artifacts, and vicious monsters, with only a few immortal survivors of the ones that saved their world from destruction - modern humanity doesn't seem to give a shit about any of it, aside from superficial aesthetic details.
The various human characters don't treat the Crystal Gems differently from other people. In a modern setting that has the internet, it should be common knowledge who the Crystal Gems are and what the Gem War was. Hell, the fact that they're the last of their alien kind on Earth should be significant enough on its own to everyone. The Crystal Gem temple should have as much if not more significance than the likes of Mecca, Jerusalem and the Vatican, especially since you can actually meet the gods, and yet there's a small generic American town behind it, utterly ambivalent to and even willing to get angry at the ancient otherworldly beings that reside within when even their ancestors should clearly have known about them. Even if they don't see the gems as gods, they should at least acknowledge what they are - ancient beings that have lived for millennia. You wouldn't interact with Jesus or Gilgamesh if they were still living today like you would your next-door neighbor.
Going beyond the human characters in the show, how much does humanity actually know about the gems? Why are there no tourists, scientists, and/or government agents at any of the gem ruins? People still visit and archaeologically research the Pyramids and Ancient Greek temples and those aren't even alien in origin. Is Sugar implying that no one would be interested in visiting magical futuristic structures left behind by an alien civilization? Why doesn't the world give a shit that Lapis Lazuli stole the Earth's oceans for a day, inadvertently damaging the Earth's environment and economy? Why doesn't the world care about the shard monsters inexplicably still roaming around, the Cluster nearly destroying the Earth (They should be able to detect something that big with their level of technology), or that the gem civilization and their rulers have returned? Kings, queens, presidents, and prime ministers should be flocking over to Beach City to establish relations Homeworld. And yet no one cares.
What I'm getting at with all of this is that in relegating humanity to mostly slice-of-life characters while focusing on the gems for the main plot, it creates the feeling that these groups don't share the same world narratively, as if you're watching two different shows. I know cartoon budgets are meager and that Rebecca Sugar wasn't writing a story along the lines of Bokurano or Turn A Gundam (where humanity's reaction to the perceived alien threat actually matters), but when so much emphasis is placed on defending Earth and how its beauty is worth protecting, only for Earth's most notable aspect humanity to get shoved aside narratively, as if the Gems are the only ones who matter, it makes the whole setting feel unbelievable, but I shouldn't be surprised considering Sugar's an ideologue more concerned with indoctrinating children than maintaining suspension of disbelief for her show.
Imagine if various human factions played a major role in the main conflict, like a corporation or government agency using gem tech for ill intentions, or a cult that worships Homeworld and is staunchly opposed to the heroes. On the less antagonistic side you could have worshipers of the Crystal Gems, and scientists enthusiastically researching every aspect of Gems and their civilization, and they'd probably help Steven establish diplomatic and cultural relations between Humans and Gems. The Crystal Gems would have to contend with not just a resurgent Homeworld, but also the people of the world they swore to protect and all the moral dilemmas that come with it, with Steven serving as the mediator between his parents' races.
You could have conflicts that result from the gems' lack of regard or understanding of humans and their needs, like taking away an artifact or an inactive corrupted gem that a village is dependent on for their survival. Going back to the Homeworld Cult, perhaps some humans would create religions of how the Diamonds were gods who wanted to help humanity ascend from their organic bodies and become gems themselves (a very twisted way of looking at the gem colonization process.) and that the CGs drove them away and condemned their world in their self-serving pursuit for freedom.