I want a game that takes advantage of a huge, huge, almost exactly like real life Earth map, while also capturing the feeling of being hunted by an unstoppable being/group.
Basically, you'd have a Google Earth type environment, and your job is to not die. You'll be chased by some sort of unkillable monster, and the only way to defeat them is to find a very specific spot on the map, which could randomly picked to be on the other side of the world. You start the game given a clue to reach a certain town/village/building, and then you have to search inside the area to find the clue to the next location. Effectively Amazing Race-ing yourself across the world. The entire world map would be filled with items you can use, but no other people.
Meanwhile, the monster is chasing you. On one end of the scale, you can have a completely unkillable, unstoppable, will kill you in one touch thing like the monster from It Follows, but its EXTREMELY slow. Rapid movement and rapid searching for the next clue are your only options against it. Survival game mechanics like having to find food, sleep, etc. are what's gonna do you in cause this type of monster will NEVER stop moving.
Along the middle end of the scale, you can have supernatural monsters ( with potential Dead by Daylight-esque killer archetypes and guest monsters ). These middle range monsters move as fast as you, potentially even faster, but can actually be temporarily killed, by enough weapon damage or trapping them somewhere. But they will get back up eventually, and their AI is absolutely gonna be smarter than the first monster. They're bigger than you, stronger than you, have unique weapons and abilities, but they're not completely invincible.
Finally, and this one's probably for a LAN multiplayer type mode, you can be chased by an evil cult of absolutely normal people, each one with the exact stats as your character. They do as much damage as you, they all die permanently, but they never stop spawning. This can lead to a variety of on the run chase events, siege situations, fun stuff.
Unlike the above two types of monsters, the cultists won't know your exact location all the time. You can encounter a few of them at any given point, but only once you find a clue, do you alert the entire mob, and they start gunning for your location en masse.
The only vehicles that really exist/work are boats to take you across islands and continents. Everything is either on foot, or bicycle.