It only took two days for modder Siegfre to crack open Baldur's Gate 3's official mod tools, unlocking a level editor that could let fans create their own custom modules or even full campaigns. A week later, and that's already started to bear fruit, with some modders sharing images and videos of playable custom levels.
The most impressive creations I've seen so far come from a channel simply called S on YouTube, with the standout being a work in progress take on Avernus, the top layer of the Forgotten Realms' Nine Hells and a significant part of Baldur's Gate 3's backstory—we also briefly glimpse it in-game through the Nautiloid tutorial and House of Hope endgame dungeon. S seems to have used a combination of assets from the Shadow Cursed Lands, as well as from the Avernus bits in the final game (like that really distinctive skybox with a floating fortress) to produce the area, which somewhat resembles Caelid from Elden Ring.
The Avernus mockup is particularly exciting because it looks very different from the base game's areas despite being made from repurposed stock assets. Also, crucially, we've got environmental storytelling skeletons here—that's how you know it's real. Elsewhere on their channel, S has a prototype level consisting of a lighthouse on a rocky outcrop, though its in-editor working name implies it might have been intended to be an Underdark location that just hasn't had the proper skybox implemented yet.
Another modder, Lotrich, has produced a small wilderness area that resembles the original party camp of the base game. Crucible Gaming, meanwhile, has a tutorial on creating new maps, as well as ones for implementing dialogue and scripting. Going off S' video descriptions, some of these modders are organizing a Discord server to collaborate and share findings related to the level editor.
Most of these demos are shown running in debug mode within the editor itself, but Crucible Gaming does show how to export a map into Baldur's Gate 3 proper. These are exclusively proof-of-concept works, and I haven't yet seen any available for download, but this is still some impressive progress for how little time we've had access to the level editor, which wasn't even meant for public use in the first place. I'm hoping this will result in a long tail of fanmade mod projects like we saw with Skyrim, Half-Life, or Neverwinter Nights.