it's all conceptual for the moment, but some of my notes/intents, since you seem interested.

Mechanical:

~somewhere around 6 to 8 weapon types, each with their own techniques (what I'm calling stragems) to learn, with special weapons (artifacts, demon weapons or just gear with special stuff built in) having extra techniques built in.

~Some kind of limit (possibly 6?) for how many techniques you can have at once, swapping them out between sessions. (This does not include techniques stapled to your gear)

~maybe a system for making pacts with demons, forever giving up technique slots to get special powers from them.

~Some means of codifying team attacks as a mechanic, since all the hypest moments in previous versions of the system were big team finishers.

~modifying the mec part system to have head/upper body/lower body slots for armor and the like, 2 hand slots for weapons and a 'misc' slot that can hold an extra weapon or some kind of special gear

~low health pools for allies and enemies alike, fights end quickly, one way or another.

Setting:

~demon hunting as a career

~Most demon hunters work for 'The Guild', an organization that builds big guild halls to operate out of over top of vaults containing ancient and dangerous artifacts.

~Some half demons and demigods work with the guild as hunters in addition to the majority human workforce

~in recent years, demon sightings and attacks have gotten far more common, as have 'exceptional individuals', the subset of humans with the right physical, mental and spiritual quirks to be demon hunters

~some folks will make pacts with demons for power, some of those people become demon hunters

This entire setting was written years before Chainsaw Man, somehow, though the demon pact thing was added just recently due to chainsaw man.

amusingly, in the time between replies the setting has evolved a little.

Instead of pacts we now have Grafts, parts of demons that you force to fuse with your own body to gain power. We also have 6 different flavors of spellcaster, split between angelic and demonic power sources.

Angelic:

~Fundamentalist (Earth, Wind, Water)

~Genesis Caller (Summoning)

~Lux Weaver (Light)

Demonic:

~Arcanist (Fire, Lightning, Ice)

~Umbra Weaver (Darkness)

~Dreamwalker (Dreams)

All with basic attacks, of course, but flavored very differnently.  An Arcanist might set you on fire, but a Genesis Caller will summon a swarm of angelic butterflies to peel off your flesh.

Plus some friends of mine have been helping me with magic items for the setting, my favorite being Charon's Obol.  It brings whoever holds it back from death once, as they use it to pay their way back up the river styx into their body, but when they die again they can't go to the afterlife at all, becoming an eternally tormented spirit wandering the earth until someone somehow destroys them. 

Stuff like that is the general vibe I'm going for, a setting where all the sharpest swords are double edged.