Playing around with all the vanilla expanded mods, and focusing on the medieval mods and various medieval themed plugins (others included) one thing that is seriously missing from any medieval game is root cellars for getting somewhat closer to refrigeration than tribals have access to.

While not quite getting to true refrigeration except at night time, a building that can only be built on soil terrain and takes a LOT of work to build obviously, plus chunks for interior supports "walls" plus wood for "shelves" and can only store food items. Initially I thought it would be cool to add a more powerful version of the passive coolers with heavily insulated walls to custom build large cellars for storing food... but can easily see it being overpowered, plus you wont get the limitation of only being able to build it on dig-able terrain; so a standalone building with limited storage targets is likely the most balanced bet here. Root cellars work by digging into layered soils like clay sediment deep enough that the local water table seeps through but being exposed to air the water evaporates quickly with the large surface area inside the hole causing far colder temperatures than surface level evaporative coolers, while wood rot is a problem with wood shelves being exposed to a dark damp environment, the upside in excess moisture evaporating from the wood further increases the cooling effectiveness -- so an upkeep of wood to keep the thing running might be in order.