Now it's time to figure out how to unfold the rest of the story. Since you can work with fantasy and horror then why not mix everything up and use both? For example, right now I'm working on the same construct and I use the same method that is used for constructing Fallout\Skyrim quests:
- Every quest block is relatively independent from other blocks so it's possible to take these blocks in and out, insert new blocks in-between and maintain them separately. They also don't revolve around main characters which makes them more or less universal.
- Every block have several possible solutions so it wouldn't feel linear and built specifically for characters to overcome and also allow multiple styles of walkthrough.
- Environment is constructed in accordance with the quest block and vice versa.
My current working concept is very rough and looks something like this:
General environment which I simply call The Rift is very similar to the Great Divide from Fallout New Vegas addon Lonesome Road (reference 1, reference 2, reference 3) so all the action will happen within large canyons and caves, there may also be ruins of some ancient buildings and so on. It should be relatively easy to reconstruct and will also allow all the necessary types of environments. Quest blocks may look like this:
A medium-sized outpost with some goblins in it (something like that maybe), they are not particularly hostile but if characters want to proceed, goblins demand to get an item from another faction living farther within the Rift to let characters go out. Possible solutions for characters is to plow their way through, to give something else as payment or to agree to get the item.
A random section of the Rift where some random monsters live, characters may Rambo their way through or sneak by. Characters may also, for example, meet some stranger that will offer them to be their guide but actually will try to lure them into a trap and feed them to its pet monster.
A section where other faction like minotaurs live which have the item goblins need but they refuse to give the item because it keeps the big bad monster away. Characters may try to Rambo everyone in the settlement, may try to steal the item or try to go and kill the big bad monster so minotaurs won't need the item anymore.
The item may happen to be the same item characters were looking for so they will have to find a new solution for goblins settlement, they may try to bring something else as payment, try to Rambo their way out, to sneak out or to find another way out of the Rift.
Something like this should be enough for a relatively short and dynamic implementation and should also allow to use different styles of action – foggy ruins with scary monster, open area for tactical maneuvering, caves for, um, caving, habitats for interactions and so on. Most of these blocks can be shuffled and changed independently and it's possible to insert or remove as many as needed. Independence from characters should allow different styles of walk-through – it is possible to make a typical girl-scout posse who will have sex all their way through (yes, monsters included), a group of Spacemarine hulks with big weapons, a regular mix of adventurers where each character may become useful at certain point and so on.
You can try to work with my concept or bring some of your own, right now its important to develop a clear set of goals and estimated ways to handle them.