One thing that would have been a lot of fun for RWBY? Grimm cults. They've been around since the dawn of civilization, and religious activity is something all humans do. The Grimm are an all encompassing menace, monsters that feed on humans for their negative emotions. They would seem like the wrath of the Gods, or something we might be able to appease if sacrifices were made. This idea of bartering with the supernatural is universal for all humans, and it would arise very naturally in Remnant. For the most part, Salem would not have directly managed these cults. Rather, she would be amused at how they rise, and provided some encouragement here or there to deal with Ozma.
At least in my own fanon, the rise of the Quitalan Empire and other great powers would see these cults reduced to the fringes. A few smaller powers ruled over by Grimm Cultists might have arisen in this ancient period, not too different from the ancient Aztecs, Mayans, Celts, Mesopotamians, and many other civilizations. But being sacrificed to monsters tends to make your people hate your rulers, and so empires whose philosophy detested this practice stamped them out, leaving them to be secret orders. Some were able to infiltrate the ranks of great empires: The Quitalans had at least one mad emperor who tried to make Grimm Worship the dominant religion before their fall, their version of Caligula. Salem might have made an appearance or had one of her agents act as an emissary to guide these cults when it was advantageous.
In the chaos following the fall of the Quitalan Empire, and many other empires like Iksander's, Grimm cults arose in response to the despair and loss and pain. In the wake of wars, these dark practitioners stepped in to try and seduce lost, lonely, and broken people into their ranks. Some were granted boons by Salem herself: A bit of magic, Grimm implants, and so on.
The rise of the Tablebreaker religion was a major counter to these cults and Salem's plans. Ozma would not have anything to do with the actual events of the Tablebreaker religion forming, but he would encourage it as it provided a source of hope against the Grimm in these times. He did the same with Pathism (the equivalent of Buddhism) as its origins were also outside of his purview but he saw the good it inspired and so he encouraged it, and any other faiths that resisted the darkness of the Grimm.
The Witch Queen of Argus rose in the wake of the total collapse of the remnants of Iksander's Empire falling, forcibly uniting the Hellenic League and making war across the sea. The Tablebreaker religion called for crusades against her and other Grimm cultists. Jeanne D'Arc herself was a believer who attributed her victory over the Witch Queen to God. A Monkey Faunus warrior and his Pathist monk ally were able to bring down a Bull Faunus Lord named "Red Boy", or the Divine Infant Great King, who had turned Mistral into his slaughterhouse for Grimm sacrifices.
These defeats made Salem become far more subtle. She would work with her cults here and there quietly, carefully, as the modern age arose. A mad king seeking immortality there, an order of priests subverted to the cult there, but all very quiet and careful.
In the wake of the Great War, many elites became disillusioned with the religions and philosophies of the old world, just as they did on Earth after the world wars. And a number of these elites formed secret societies, becoming Grimm cultists. They used and abused the lost, the poor, and the desperate in their macabre rituals because they had lost faith in humanity. A number were actually found out and arrested, but legal battles with the rich and powerful are complex affairs. At least a few of them got away thanks to their riches, influence, cleverness, or luck. They still try to make contact with Salem, still make sacrifices in the hopes for power, and still practice the old ways in secret.
(Obviously inspired by H. P. Lovecraft's stories.)
This would be seen most dramatically when the Kauravas overthrew Pandu. They didn't overtly practice their Grimm rituals in public as that would lose them influence, but they kept them up in secret. The reveal of this by Team AARN with Ozpin's help allowed for Arjun to rally the people and overthrow them. At the time though, they would be seen as whackjobs who believed that the Grimm were ruled by a goddess, something in the modern day would be seen as insanity. Ozpin would suspect the truth and try to learn more, but the surviving Kauravas went deep underground, and Lionheart helped cover them up.
At least a few in Vale's government could explain why the city fell so fast and why Cinder and her agents were not found out. Lionheart and others like Hazel or Tyrian being contacted by Salem though such a cult would deepen their characters and the lore of the world. And of course, it gives you more possibilities for villains.
At their core? Most of them would be motivated by despair and nihilism. There is no defeating the Grimm, there is no saving Remnant. There is only trying to gain power by appeasing the ravenous darkness threatening to consume us all. Some like Tyrian would feel serving Salem is truly being righteous, is truly fulfilling (and would also be sociopaths). Some might even believe they would be reborn as Grimm when they died. The possibilities are vast.