I've pretty directly stolen the Roman Empire and feudal France for homebrews before. Shamelessly, even. I used GURPS Rome to help flesh out my Rome analog. But I'm not interested in recreating Roman folklore. I'm more interested in saying things like "these people have a culture and government similar to Augustan Rome, but with some Byzantium thrown in, and what does that mean given D&D's tropes?" than I am in the notion of making D&D fit to Rome.
In my mangling of Dragonlance, the Ergothians currently have a Roman/Byzantine culture and government where the Church of Light (modeled very heavily on medieval Catholicism) is influential and established by the state, but the Church of the Twilight is fully legal, tolerated, and its faithful are especially useful to the state. Too useful to throw away. European Jews were sometimes in similar positions in Early Modern European governments. But the Church of Dark (Takhisis and her people) is modeled more on ancient Near Eastern religious practices and represents a more faithful continuation of how the Church of Light (Paladine and company) used to function when they were all a part of one religion with a single church. The Light developed its present structure after it sided with Solamnus in his rebellion against Ergoth. The Dark were the traditionalist party, the Twilight neutrals that ended up hated by both. Once I decided that I would model each religion along the lines of a real-world example, their interactions and history developed in the fantasy context.
Also, all three churches include elements of their dogma that could be considered pure evil and generally good. I wanted to go very far away from the sort of moral absolutism that the DL cosmology seems to imply. Paladine is not all good and Takhisis is not all evil. Rather they both define these terms exclusively in reference to their portfolios and pronounce the other evil. Within the three religions that divide the pantheon are other ways to be "good" or "holy" that the gods therein have agreed are ok for followers of X to abide by. About the only thing they agree upon is that Heathenry (roughly druidism, I removed the nature gods and the magic gods) is absolutely evil and must be eradicated at all costs. The formerly-Neutral gods are a bit less serious about this than the rest and tend not to be aggressive about it outside their own communities. Heathenry, to them, is wrong for their flock but not necessarily a universal evil.
I'm sure I intended to answer a question before I got distracted by my homebrewed Dragonlance, but I can't remember it now.