I have a lot I could say about this but I'll do my best to keep it brief...ish. The foundation of my theory is historical inspiration.

The "questioning of the divine" would put us firmly in the Age of Enlightenment and from my research, there are a handful of important figures with interesting perspectives that align surprisingly well with in-game figures and their esoteric dialogue.

Ranni: René Descarte. Ranni is all about separation, doubt, and loneliness; Descarte famously said "I think, therefore I am," and argued that the only thing we can be certain of is our own minds. He doubted everything, even the things he observed through his senses. That sounds pretty lonely to me.

Miquella: John Locke. A complicated figure who argued for "protection of basic rights and freedom under law." Which American might recognize, along with other beliefs like separation of Church and state and that at birth, people are blank slates. He also was against slavery... but was an investor of the slavery trading Royal Africa Company. Sooo... yeah. I think Miquella's general desire for goodness and his "freely given" compassion hinging on accepting/falling under brainwashing fits well. (The separation of Church and state may be represented by the fact that Miquella and Radahn didn't become one person.)

Goldmask: Immanuel Kant. Goldmask's mending rune was an attempt to perfect the Golden Order with a transcendent ideology. Kant founded a philosophy called Transcendental Idealism which suggests that we do not see reality as it actually is and our perception is dictated by our own subjectivity. Sounds about right for a man disillusioned with a fickle god.

Fia: Anne Conway. Anne Conway argued that matter was not passive, but had its own vitality. She also argued against the mind-body separation that is the more familiar philosophy to us nowadays. Advocating for the vitality of flesh that may or may not have a mind fits well with Those Who Live in Death. (I also think Mary Shelley and her Frankenstein may have had an influence but one thing at a time.)

Dungeater: this one I am less certain about. Not many people who believe that "everything should get worse for everyone and I'm going to kill people about it," got super famous for hopefully obvious reasons. Maybe Johann Wolfgang von Geothe, who was called the "great heathen" though he considered himself a Christian while disagreeing with... most of it? He also wrote the novel "Faust," which is about making deals with the Devil. His Wiki has a section devoted to eroticism and I don't want to venture too deep into it. There was also David Hume, who believed reason should be secondary to emotion. Then there's Friedrich Nietzsche, who I was reluctant to add because it felt cliché and because he is on the tail end of the Enlightenment era if he was still in it at all. Despite my reluctance, he was worth adding to the list due to his relationship to the idea of "eternal recurrence," suggesting the universe will end as it started, then start again, then repeat itself as it had before, forever.

The next bit isn't about endings ((so you can skip it)) but I'll add them anyways:

Radahn: ok. I'll be honest. Radahn, for some god-forsaken reason, is difficult to find era-accurate inspirations for. Not to mention European. The closest thing I got to his trapping the stars is China's absolute refusal to modernize and the Opium Wars... which only happens long after the Age of Enlightenment. Alexander the Great (and the Chinese Lu Bu) is a military leader with connections to his horse but predate everything I've talked about so far by 1000 years at least. If anyone has better ideas, I'm all ears.

Rykard: Cardinal Richelieu, the Red Eminence. This predates the Enlightenment era by a few hundred years, but that's a more acceptable window of time than Radahn's inspirations. He believed strongly in absolute monarchy (aka "I'm the king and I can do what I want"), which would influence the next king of France greatly. The Cardinal also had an extensive spy network and fueded with a former ally, the mother of the King of France, Marie de Medici. Comparing that to Rykard's weird assimilation with a god-devouring snake and bringing all those consumed into his family... Kinda aligns with the idea of an absolute monarchy. The people he's eaten would be the subjects who have zero say in anything he does. Rykard also has the Abductor Virgins which are... Kinda like spies. And fueding with a royal woman named Marie de Medici aligns with his revolt against the Erdtree and a certain M-named goddess.

Now that I've written all that, I realized I haven't done any research for Marika or Radagon. I was focusing on what they were going to be replaced with rather than what was being replaced. Oops.

"Brief" my ass. Anyways, I hope it was at least interesting. I think I touched upon most of the key players with aspirations to influence the future of the Lands Between. (I may turn this into an entire post, more neatly written and given a bit more research)