Btw none asked about maggots? It's ok, understandable, was not insulted or anything, since you can't possibly know that it's the best underground suggestion. Don't worry, I'll explain anyway.
"Maggot infestation" - not like separate entities or a new enemy. No collision boxes or anything, no AI, nothing really.
These gifs are the closest thing I found on the internet to describe what I am talking about. The search was not pleasant.
Technically, TFP only have to draw and animate a static picture on a block, fade them in and out or change the "gif", as the block's infestation levels diminish or increase, and "write a script" about how they will be "spreading" from block to block. Imagine a tunnel/cave full with these "gifs" on each block on the walls/ground/ceiling of the cave. "Make the game horror again" (and less scooby doo). Even a shoddy fade-in on a newly infested block would imo look decent in game.
Players will naturally start getting infested as long as they walk on those blocks, stand below them (particle system falling maggots) etc. At first stage the infestation will be reversible with time with some dellusions or those A1 sounds, at 2nd blah blah, bleeding whatever, you will be able to see animated maggots that will be eating into your hands, at 3rd death (which is rewarded in this game but that is a whine for another time). A machine could repel them in a range etc. Certain blocks could be maggot hives that the player will have to deal with with various means. When players destroy infested blocks maggots spill on the player or nearby blocks worsening the spread etc. There is a lot of room for dynamic elements here. And a piece of cake to implement afaik.