Okay, just a quick clarifying question: are the characters expected to advance and grow, or is the theme of the campaign or set of stories run in the same world more like a set of CoC vignettes?
No advancement, it's 1 adventure then all new characters. For all intense and purposes imagine each adventure is a separate movie, whole new cast. The only difference is that I can put it in a fantasy setting, rather than in the modern world, purely for the sense of flavor and it allows various things to exist there (demons, ghosts, magical set pieces) without that being tied to The Big Event.
A lot of your suggestions are in the direction I've been thinking. (PS I love the idea of the portals being people, living vessels. It jives with other thoughts I've had on the topic.)
For instance the Good gods are just silent, and I'm thinking that they perished/went insane/fully locked in the process of preventing the Old Ones from stepping over into this dimensoin and shattering it. The only Gods left are the darker ones, but it ain't easy out there because there's so few worshipres to go around now, that inciting war or trying to take over places is actually risky. It has to be subtler because the resources of souls is a lot smaller. And lots of little fiefdoms sprout up with little protector gods that can enact a steep price, or fiends/liches/bla, that there's not so much prayer to go around.
The stark on-the-ground issue is that the Far Realms breach is more like a radioactive waste spill; there's mutative energy out there. Especially where such creatures congregate, they're walking wounds in reality that taint what they touch. So travel between Havens is dangerous not simply because it's a wild world out there, but you're possibly exposing yourself to being tainted even by simply walking through a polluted area..
Faced with insanity and having one's body melted and transformed, or suffer both in life and in death eternal, many turn to necromancy/the undead. The undead are far more pragmatic. Give us your blood, your dead, sacrifice now and then, and you're safe, we leave you alone. Or become a ghost, it's lonely and cold, but your family can be together eternally, and you will remain
you. Mostly.
I would like to add Fae in here, but I think it's getting a bit crowded, and the Fae kind of dip into madness and nightmare/dream too. If I pulled one of the other factions out, it would be easy to contrast the Fae as the wild
but natural, the manic fever dream of life, to the unnatural gibbering nightmare of the Far Realms, but I think I have enough as is.
I definitely want to do a haven/town that's fully sworn allegiance to Hell. Which means that everyone living there is already suffering, their sins visible on their flesh, tormenting them in a lesser degree, and they can barter with fiends by committing more egregious acts, thus putting them deeper into a debt of sins. But given this price, the Haven is prosperous, because Hell is making sure to keep this prime soul factory as lucrative as it can.
The more I think, the more this really is very post-apocalypse. Your dystopian cities of brutal warlords, your wastes full of mutants, your scavengers just trying to scrape together. What initially inspired all this was the Scarred Lands, which is a fantasy world in the wake of a war between
evil, nasty Titans and the Gods, the war and aftermath polluting everything. Where clerics of good and evil worked together because damn, dude, we gotta get through this.