Acid rain should affect farms (If it doesn't already) and introduce new farm plants

[ButterStuffed]

Acid rain starts Wormwood's blooming, so it feels appropriate to have farms be affected as well.

Adding Growth Formula to all farm plots underground would be a great way to encourage people to bother with acid rain and could pull some of the less popular farm plants into the limelight without making the other options obsolete. It would be even better if this is applied to Wormwood's wild plants as well, similar to how regular rain is the only way to water his wild plants.

It could also be very interesting if new species of farmable plants could be bred under the acid rain. Either twisted by the influence of nightmare fuel, or only to be found naturally cultivating usable seeds deeper in the caves. Pomegranates could transform into Pomegranite, Raspberries and Eldritchberries could grow around fully formed shadow fissures, Persimians could grow near splumonkeys, the possibilities are endless.

On the topic of farm plants, similar things could be done with Brightshades. They could interbreed with nearby farm plants of the nightshade family to produce unstable breeds that could be used for some nefarious purpose. It would certainly create opportunities to interact with them in ways other than combat.

[Pet Rock]

Acid rain boosting crops would definitely add more incentive to turn on the rifts. I also really like the idea of pomegranites. Maybe these could act similar to stonefruit, and could require gunpowder (since granite is durable) but result in many pomegranate seeds.

One Idea I had for a new crop is the glumflower (rotten stage on the right):

Normal seeds in crop plots can be converted into glumflowers by acid rain. Glumflowers will only grow during acid rain. At their final stage they produce miasma and can be picked for a glum bloom (gloom) and glumflower seeds depending on their happiness. Glumflowers gain happiness in the dark, when talked to, when fed pure horror, when acid rain is active, and a lot when nightmare creatures are nearby. Glum blooms can be eaten for temporary neutrality towards basic nightmare creatures or be thrown to create a cloud of miasma that slows and damages enemies. If glumflowers rot, they will pour out shadows, resulting in a rift size cloud of miasma and a few infused shadelings and hostile nightmare creatures. This will continue for a couple minutes before they retreat into the ground. To avoid chaos, only one glumflower in a certain radius will summon shadow creatures. This mechanic could result in (literal) farming of pure horror/Nightmare fuel without finding rifts. Note that glumflowers are still pickable during their rotting phase and their rotting phase is also dependent on their happiness level (more happiness=longer rotting). Wild glumflowers can be found anywhere but will permanently rot if in the ruins.

[Moongo]

that's actually a really good concept!
it would make cave bases more worth having as well