Apart from the zombies, the world of cata doesn't seem to age much. Maybe make it so that after an amount of seasons, grass tiles next to a bush have a percentage chance of spawning a bush? If implemented, this could make the passage of time seem much more tangible, with the wilderness encroaching on previously "tame" land, as well as make roads way more valuable endgame once you have a multi ton vehicle with rams (I can currently handle 80mph offroad for hours with only yellow damage to 2-4 out of 7 front rams).

If this furniture aging (? I think bushes and trees are furniture, might be wrong) system was implemented, it could also facilitate tree spreading. After an amount of years, bushes that are next to tree tiles could get a percentage chance to turn into young trees, which would then grow up and continue the cycle.

We currently have grass and dirt tiles. We could instead make grass a furniture (low durability and can be hit by wheels, if you run over grass a couple of times IRL it dies) that spreads as described above to dirt tiles to further facilitate the feel of wilderness retaking the land.

I have done my best to try to figure out a way to prevent any of this from happening too quickly. The way I see it working can be outlined like this:

check is made monthly, so percentages that seem too low here might actually be too high

dirt to grass: takes 2? weeks for a 25% chance in the spring-summer, only next to grass tiles

grass to bush: takes 5? months for a 6.25& chance in the spring-summer, only next to bush tiles (maybe a 1.5% or so chance on non-adjacent tiles?)

bush to young tree: takes 1.5? years for a 6.25% chance in the spring-summer, only next to tree tiles(separate types of tree have different times/chances/seasons?)

young tree to mature tree: takes 2-4? years for a 12.5% chance in the spring-summer (low chance because close mature trees steal nutrients and light from saplings, hindering their growth) (might already happen, I haven't watched a single tile for years to check)

If you want to take it further, you could possibly track when a 2x2 of trees is formed (by checking if a single tree tile has 2 trees around it, then checking if those trees also have two trees around them, then by checking if those tiles overlap with the previous checked tiles, ensuring it is a 2x2 and not a 1x4 or something), and give it a tiny percentage chance per year to turn into a giant tree, with stats WAY too high to ever be knocked over with a vehicle, even a super heavy one with rams. The tree could go up a Z level and provide an insane amount of wood if chopped down, or a starting point for an elf-style treehouse. (This one is purely the logical extreme to show what the implementation could lead to, creative direction notwithstanding)