Most of the world in the Cataclysm is lush forest and fields. I do not like this and plan to change that and luckily it looks like it may be easy to implement.
As a start, I would add toxic varities of the forest and swamp biomes: More dead trees, replace water puddles with sewage and even some toxic gas vents. The Cataclysm meant death to lots of organisms, including humans and also plants.
They should still be semi-rare though.
Something which should, in my opinion, be relatively common would be dead and warped foliage. So basically add a variant of forests and swamps where there are more dead trees and some water is replaced with sewage with a relatively low chance. No toxic gas vents.
And something experimental which would fit into the current lore really well: Warped foliage. Basically distorted dead trees, think of branches stretched into different directions and strange patterns. This is purely cosmetic.
Maybe also mutate bushes and fruit trees, so they bear pyramid-shaped fruit or something.
My clear focus is to implement the dead and toxic varities and also the warped dead trees, not the mutated trees and bushes and so on. This should basically encourage the player to scavenge more and that living off the land will be harder.
The implementation would basically be modifying the chances forests and swamps can spawn and add some dead/toxic varities, this should be relatively easy™.
Since this might be really harsh, this is just an idea for now: Add a variety of dead/toxic biomes, e.g forest_dead_80, where 80% of the stuff in it is dead, also forest_dead_70 and so there will be a smooth blend between dead and mostly alive biomes. However, this is meant to be used when a significant amount of biomes are dead or toxic, so this would make New England truly apocalyptic.
Might be a good idea for a challenge mod even if it will never be in the “vanilla” game.
Another evil idea which I would not implement (yet): Leech plants looked really interesting. Maybe let them infest a few forests.
Edit: Maybe also something something radiation so it actually gets used more often?