Modding is now officially planned. It's being seriously worked on by Evil Factory, who worked on the pretty cool LuaForBarotrauma mod.
We already started testing custom maps and the Steam Workshop, and things are making some solid progress.

Modding will happen through UnrealEngine tools, which means that you will have access to all of the tools used to make the game to expand or add content to the game. We don't exactly know how much of the original source of the game will be made available to modders, but we will try to make sure the modders have enough freedom to add what they want to the game.
To that end, I'm also looking for modders to early-access the modding, I'm looking for people who want to make mods (not play them). So if you have experience in UnrealEngine, or you are interested in learning, if you have mods before or you want to start, feel free to get in touch on the official Discord.