I love the 19th century's Gothic novels like The Manuscript Found in Saragossa or Melmoth the Wanderer. Love the idea of proper magic, witches being dangerous, Lucifer as active player, corrupting souls and commanding agents, the Christian faith being a shield against darkness, which is everywhere, the Spanish Inquisition fighting the good fight, cursed places abandoned for a reason, forbidden knowledge, supernatural creatures, that sort of things. Needless to say it would make a great setting, but how do we get there?

I start with the main character, a monk, perhaps, armed with faith and potent prayers fueled by that faith but full of doubts that can be exploited. Basically, a character development fork: a man of God bringing light into darkness or an occultist who abandoned God's light, trading it for forbidden knowledge. Both character types would communicate with powerful demons - dukes of Hell and such, but in different ways. As you can see, now we're defining the setting, both mechanics and "lore":

- to start with, the demons and most "magic-users" can't be killed, they are far too powerful; you can only bargain, protect yourself, play them against each other, maybe banish from your presence to piss them off even more; thus the focus isn't on demon slaying but surviving in a dangerous world most people are unaware of.
- you use combat (or speech/stealth) against the human minions or human enemies standing in your way, like the Inquisition if you turn to the Dark Side.
- such a setting would be perfect for relics, both Holy and Unholy.
- skills like languages (reading ancient books, communicating with demons, etc) and rituals (from exorcism to summoning), derived stats like Faith and Sanity, etc.
- magic would play a large role: prayers, words in God's own language, signs, magic circles & pentagrams, rituals, spells and incantations. You'd acquire them from archives, NPCs, rituals, etc. Too much "hands-on" Occult knowledge would weaken your Faith and thus the power of your prayers and Holy relics, just like too much Faith would weaken your dark rituals and incantations (i.e. it will be hard to play a hybrid).

Now that we have some gameplay basics, it's time to figure out how such a tale would start and end as these two story points would define the rest. The ending would give us both the direction and options on how to get there. The start of the game would set up the tone and introduce the first choice. Etc.
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Such a game would be perfect for a class-based system, word-based magic (like runes but with words, you learn new words rather than spells and string them together), gain new abilities via rituals rather than level ups, kinda like Bloodlines where you discover that there is a whole different world (of darkness), flintlocks and melee, single-character with summoned allies (animated suits of armor, raised dead, lesser demons, etc).

Так сказал Винс...