I'd love to see further expansion on the mushroom system in order to bring them out of a fully foraged status into something that can be cultivated and farmed.
There are a lot of ways this could be accomplished, with differing levels of complexity and modernity.
The oldest recorded cultivation method for mushrooms involved chopping logs, scoring their surface and leaving them to sit near trees and stumps already growing mushrooms for a year or more until they became inoculated.
Nothing new would really need to be added for this method except the interaction to score the wood to prepare it. Its a slow but historically effective method, and the inoculated logs could then be moved back to a players base for continued use, continuing to fruit for a few years before production stops, similar to the new berry mechanic but faster, since a log like this generally only lasted for about five years.
More modern practice could involve gathering spores with parchment and making plugs that could be added to logs or trees (drills could be added for making holes, but just using the knife would work too) and then sealed with wax.
For non wood growing mushrooms, substrate could be made with dry grass and grain that are boiled for sanitization and inoculated with collected spores. Substrate would be converted into mycelium blocks over time that could also be broken up and used to inoculate new substrate rather than left to fruit.
Collecting spores could be as easy simply interacting with a given mushroom using parchment, or could involve mushroom life cycles under which the spore print would need to be taken at the right time in the process.
Overall I just think it would be cool to be able to grow my own mushrooms instead of only foraging them.