Baldur's Gate 3 mod tools won't be as extensive as Divinity's: 'We're not a tools company,' says Swen Vincke
The RPG Larian made before Baldur's Gate 3 included an extensive mod development kit which could be used to build whole new campaigns with custom maps and NPCs and quests—something like Bethesda's Skyrim Creation Kit. It's part of the reason I voted to make Divinity: Original Sin 2 our 2017 Game of the Year, but sadly, the complicated, crash-prone tools didn't lead to a thriving custom campaign scene for Original Sin 2.