I'm working on something new once Dark Citadel TTRPG is finished: Heresy and Doom, a boomer shooter inspired post apocalyptic action survival horror ttrpg.  Rage against the dying of the light using heavy firepower set to screaming metal guitars.  Here's some details about what I have planned:

Inspirations: Doom, Quake, Hexen, Dethklok, various kinds of metal music, pop-culture demonology.

Story So Far...

The Last Days are here.  Gateways to hell have opened on every continent, spilling forth the legions of hell.  Following a general military defeat, humans retreat to defensible fortresses underground known as Enclaves.  Each enclave is a self-contained and theoretically self-sufficient habitat.  Humanity hasn't fully fallen into despair yet and sends out Doom Rangers, specialized teams of explorers, survivalists, scholars, and warriors.  Sometimes the scour the ruins of earth for some sign or method of salvation, other times they conduct guerilla warfare raids on demonic strongholds.

Basic System

A revised and improved take on Dark Citadel's dice pool system.

Gameplay Types

Investigation and Exploration - Search for clues, resources, or other things in an effort to shore up your knowledge base, the material resources of your Enclave, or the allies you can call on.

Savage Battle - Use big guns, big swords, chainsaws, nova cannons, rocket launchers, and devastating sorceries to send the legions of hell and their various servants back to Hell.

Defining Feature of the Game

Guns, lots of guns!

Powerful magic that might damn you forever!

A setting that requires you to cling to any light or hope in the face of seemingly impossible odds.

Magic for Heresy and Doom

How does magic work?  The caster channels Hellfire, which is the aethyr that forms the spiritual essence of the beyond, corrupted by the demonic forces.  Doing so causes traces of Hellfire to infect the mind, body, and soul of the caster in the form of Corruption and Dark Marks.  Corruption is a numerical value and Dark Marks are the consequences of accruing too much corruption.  If you get too many Dark Marks, your soul is pulled into hell and transformed into a demon.  

Magic is:

Spiritually corrosive

Physically mutating

Mentally addicting

How does magic feel?

Profane, shattering, invasive, powerful, borderline out-of-control

Sight: Blood red flames, black mist/energies

Sound: Wails, roars, thunderous cracks

Smell: Sulfur, decay, something like poisonous rose blossoms

Feel: Searing and slimy, like boiling puss flowing through the caster's veins, ecstasy and pain

Demonic Monsters - The infernal legions manifest.  Physical forms and supernatural powers vary by a lot.  The truest sign of a demonic presence, though, is the Shroud, a distinctive mix of fear, rage, pain, and dark desire felt in their presence.  Their bodies can die as normal but the mind and spirit return to hell.  Can be re-summoned to wreak havoc.  Essentially immortal.

Undead Monsters - Corpses reanimated by infusing them with Hellfire, formerly alive but now fully converted to the cause of evil.  Ignores unconsciousness, ignores bleeding, and becomes inert but not dead when damage or other effects would make it dead.  It is only fully killed if head or chest is destroyed, otherwise it continues naturally healing.

Construct Monsters - Machines who have been corrupted by the Scourge, a demonic computer device that infects anything capable of true artificial intelligence.  Enclaves can only use unintelligent technology because of this.  Ignores unconsciousness, ignores bleeding, shuts down indefinitely when "dead."  Body parts not fully destroyed can be repaired and destroyed parts can be replaced.  Does not naturally heal.

Beast Monsters - Non-human life forms mutated and corrupted by exposure to the demonic energies of Hell.

Human Monsters - Humans who have been corrupted by demonic influence.

Cybernetics

Characters can replace body parts with cybernetics.  These parts do not naturally heal but they have natural armor values and may be more effective than biological parts.  However, a living body has a Cybernetic Capacity rating that dictates the maximum amount of cybernetics a body can have before it starts to suffer ill effects.