Act 1: Player is a sneaky ratman or ratwomen breaking free from pagan slavers, finding all members of their enslaved family, freeing them one by one and bringing em home.
Act 2: Builders raid the rat village kill some ratpeople and steal an artifact of immense religious importance. Player gets tasked to recover the artifact and follows the builders to their outpost. Player somehow gets hold of the artifact and brings it back to the ratpeople village.
It is decided that the infinite growth of the human settlements endangers the rat people as a whole and that something has to be done against that. But knowledge about the enemy is scarce and player is one of the very few, who can speak "human" (actually pagan, but close enough). So player gets send to infiltrate the next city with a tinkerer (maintains the player's gear) and a granny which happens to be the only one available who can read and write human (she is supposed to teach player that skill).
Player makes camp somewhere reasonably safe outside next non-pagan human settlement and does some reconnaissance and resource gathering missions.
As new villagers seem to mainly come from an actual city, that city becomes the new exploration target. The three ratpeople travel to the city, make camp somewhere safe outside, find a way in, find a safe place in the sewers and make camp there. More reconnaissance and resource gathering missions happen. It becomes more and more obvious, that resistance would be futile. The humans are just too many.
The three ratpeople travel back to their village and report to the council of the united ratpeople tribes. The decision is to mount a covert resistance with the intent to delay the human expansions until some more sustainable solution is found.
Act 3: Player has most experience in human-controlled area, so it is them who gets to do business in the big port city where all the support from the human's mainland arrives. From the sewer base, reconnaissance, sabotage and support missions to strengthen the local gangs (which are hostile to each other) are done.
This is the longest act. Shittons of different missions are done by the player. The city becomes more and more chaotic. Sometimes the player sets stuff on fire. Sometimes they steal extremely valuable items, sometiems they frame officials - or assassinate them... Places get heavier guarded and look more and more desolate.
Gang contacts yield opportunities for missions too.
Message arrives from home that a solution might have been found. Player is to find out of the whereabouts of an ancient artifact and recover it. Player discovers in missions that the artifact has been brought to a keep in the human's homeland. So player needs to somehow travel to the human homeland with one of humans' big ships.
Player is in a huge port city in humans' homeland. Finds a new hideout in the sewers (seriously the only place humans don't like to lurk around) and does reconnaissance missions to find out where the keep is. At that occasion, some opportunity sabotage and assassinations can be executed too. Player finds out where teh keep is and travels there.
Player finds a way into the keep, finds out where the artifact is, finds out how to get through the mundane and magical safety and security measures and steals the artifact. The artifact immediately takes ownership of the player and commands them to collect the other artifacts of its set. Player travels to other places and does missions to retrieve the artifacts. Player immediately knows where the artifacts are which aren't protected but also has to do missions to find out where the protected ones are kept.
The artifacts are reunited, player does the ritual to combine them and the entity within them demands a new worthy body and a kingdom to rule over. Player finds out where the humans' king is and gets closer and closer, finally touching the king with the artifact. The artifact disintegrates, its former tenant moves inside the king, the possession of player ends and player immediately kills the king (there is a reason why that entity could have been captured in the first place) and the entity is gone (probably just died like a normal person) - but so are the artifacts.
Player travels back to port city, finds a ship headed towards home, sets sail - and arrives at port city it rat peoples' homeland. the city is more or less like it was when player left - just a bit more run down. Player travels back to their rat people village and reports.
Act 4: Missing other options, the rat council decides to continue the covert resistance against the city dwelling humans. The pagans have "joined" the resistance mainly by raiding weaker frontier villages. But they also raid more in general now. So the player is sent to investigate, why... But that is content for the first DLC.
Semi-open maps (maybe even a whole village as "open world" but no actual open-world-sized open world).
Mechanics that interact with eachother waiting to be exploited by players in (un)anticipated ways.
Real shadows, real sounds, some parcour (plus sliding down ladders/pipes/ropes, hanging down ledges and procedural free climbing).
The blackjack, the rat's claws, the rat's teeth, the broadhead (but none of the other arrow types).
No mines or grenades in most missions (just an unlimited supply of stones for distraction).
No potions (maybe holy water to throw on the ground or directly at undead), healing by eating (very slow regeneration).
Lockpicks with various designs (the minigame is the same but you don't have to choose the pick or change it in the middle and for some locks a purely cosmetic tension wrench is used for immersion's sake too).
Missions are selected from the main menu, from a menu when using an exfiltration point, from a menu that pops up when frobbing a map in one of the hideouts or camps - or by talking with other people. Where game world consistency allows for it, missions are playable in any order. Once-completed missions can be replayed at any time.
You can save whenever you want like in TDM and there are autosaves triggered on mission start/completion, map change and at mission-specified events.
Tech & development:
Unreal Engine 5, SpeedTree, prerendered AI-generated voices (they are good enough now), game mechanics from TDM plus a bit more procedural parcour, better physics and a dialog system. The stim system is rad and has to be in there too. Missions are mostly like small-to-medium-sized TDM maps. Assets are made in Blender, maps are made in Unreal. If something is needed that is missing in unreal or Blender, a plugin for Unreal or Blender is created... As it is impossible to fund such a niche game completely in advance, the game is developed as early access with modding as first-class citizen from the start and all plugins are released as FOSS as they are made (hoping to establish a Bethesda-style long-tail business due to people still buying the game ten years after creation because they want to play the mods).