~~~~~Okay, incoming head cannon~~~~~
Some people have been talking about Salem being misinformed now, thinking Ozpin is dead. I doubt it. Maybe neither Ozpin nor Salem can die. I mean from the way Salem reacted to Cinder's answer on this matter, she seems to know that Ozpin isn't dead. She wonders aloud "what are you planning?" Who else could she be addressing if not Oz? And yet the only way she could absolutely know for sure is if Ozpin can't die. Maybe him and Salem go through reincarnation? I'm not saying they're gods but magic has been revealed as a thing in this world. And plenty of RWBY fans of theorized that Ozpin is the wizard from the Maidens story and the real story of how he gave these girls powers is a lot more complicated. Also Ozpin said he's made more mistakes than any man, woman or child in the world. You'd have to live pretty long to make that many mistakes, no?
Anyways, here's my thought: a long time ago, Salem and Oz somehow mastered magic beyond the realms of what was thought possible. They became so adept that they got cocky and began to experiment. (In this head cannon, they're like young teenagers and this is like thousands of years ago and the world was very different. Also they were like totally different people then of course. And together either as friends or romantically.) And one of their experiments goes wrong--take your pick from the usual scenarios: they realized they could fuse their bodies with magic and make themselves more powerful/ the pandora's box trope/ they cross into alternate dimensions and come back different...etc. There you go.
I bet you're thinking, why would they ever be together romantically?? These are two kids we're talking about here, not the characters we see now. And everyone wants to know why Salem wants to destroy humanity and watch Ozpin burn...Maybe it's personal. Or maybe it started out as personal and then the nature of the magic she absorbed/abused began to fuck with her head. Who knows?
Anyway, Ozpin tries to stop her from going insane and destroying things. She's like no, fuck you. I know the concept of magic vs. dark magic is overused but lets just say that Salem absorbed/became influenced by dark magic and Oz by good/light/whatever-you-want-to-call-it magic. I mean that would explain why she seems to produce Grimm in her backyard, has control over Grimm, is able to communicate with them and seems to be the expert on all things evil/scary. Just look at her. Every detail says personification of evil. While Oz's motivations haven't been revealed and he's keeping a lot of secrets, we know he's trying to stop some evil bitch from ending the world. So we can assume he's a good guy, even if he's manipulative/secretive etc. Also we know he feels remorse for a lot of big mistakes apparently.
So maybe its over love/heartbreak/something else/Salem going crazy but for whatever reason, they start fighting. And because they're sooooo powerful and they have abilities they shouldn't have, they start changing the landscape a bit. You know, like in Naruto Shippuden when Hagomoro and his brother go up against their mom and in the process, permanently scaring the landscape while accidentally but significantly reducing the population. It would be like that. Maybe the moon being shattered now was a result of that fight.
Maybe, as a way of causing Oz further distress because he hates seeing people die because of him, Salem creates Grimm? Who knows--we don't know where Grimm come from, its just a thought. So after the fight is over (lets suppose Salem is temporarily beat back), the surviving continents and population are eventually called Remnant because its a remnant of the world that once was??!
I know what you're thinking, what about the first episode which opens up with Salem talking about the beginning of man--I think that would be post-Remnant. Also, maybe because Salem's mind is fucked up, that's all she remembers? Who knows, I'm just throwing ideas out there. Maybe Salem's version is right. If you scratch the whole idea about Grimm..I think my theory still works?
I bet some people reading this are like "that would suck I don't want any character to be that OP." Yeah, I know. Maybe as a tactic during the war with Salem that's lasted for centuries, he decides that he can't do it alone. And also, he thought it would be wiser to break up the targets--instead of him being the main target all the time, there would be several other people he could count on which would even the odds of successfully defeating her in battle. Maybe he wanted to find a way to protect people while still fighting her--so he needed someone powerful enough to do that. Hence the Maidens! (Also if Salem did create Grimm then he might create the Maidens in response to that, to protect people and even out the numbers--I feel like extremely one skilled warrior with Maiden powers=100 Grimm)...or something like that.
I bet someone has thought "well if Ozpin did divide his power like that, that would leave Salem as infinitely more powerful than him" But if creating a new species like the Grimm reduced her power too then they would be evenly matched...maybe.
Did any of that make sense?
I know a lot of this is conjecture, I'm not saying there's substantial proof. I'm just throwing ideas out there.
Hope someone replies because I kinda put a lot of thought into this. :(