I have an interesting idea, which will be of interest to developers and to all game community.
Stone.
Stones is the only one in-game resource, which is not used as well.
It's used at Boiler, Burner Mining Drill and Stone furnace. Also, it's used at Steel Furnace, Electric Furnace and Oil refinery as Stone Bricks. And thats all, and no any subsequent uses.
At the same time, other resources are used at the all stages of the game.
How about idea to make new in-game recipe - Fundament (which contains Stone and Stone Bricks or something one). That recipe must be easy, available at first steps of the game and not mixing with other resources, like copper and metal. And it must be balanced to withdraw stones from the game, like others resources. Also, it should be good for the player, forcing him to build small factories that would make new «floor» for main base and for all outposts, to prevent sneaky attacks.
As you can see at my
B. and C. base with Fundament near walls
D. basis of our base is fully protected
With that you can add new types of Biters or expand possibilities of current Worms. In the following context I will talk about new Biters with ability to burrow and move under ground with one purpose - find weaknesses inside the base (A) and destroy it from the inside. They can move under ground and under Fundament (B and C protection not productive to avoid easy way), but they can't dig out under last (D). To prevent more stacks under ground Biters under your base it can resolve something like they moves constantly with trying to find a hole in the defense and never stop.
Anybody play Duna? Main idea comes from it.
They also can hurt buildings or other structures from under, if these objects are not on the Fundament.
To prevent grey mass «floor» at base, textures of the Fundament should be different as much as possible. With random cracks on it, with grass and bushes that grow through the Fundament, that will be nice!
P.S. dont think a lot about Pipe-to-Ground and Underground belts conflicting with Fundament, not! It works as well as on other types of land surfaces, with one exception - Biters shall not pass!