I'm really hoping Larian come through with the mod tools; I'm hoping we don't get half-baked tools, even though that's where I'm setting my expectations. Thankfully, most anything more than what's already available to change in the game would expand the capabilities of modders.
But the potential is huge. The engine from D:OS2 has the capability of a DM Mode. With appropriate rules tweaks (and, preferably, the ability to do so in-tools rather than external mods), a level editor/scenario creator would basically make BG3 the new 5E virtual tabletop.
There's much I could ramble and speculate upon, but suffice it to say I'm very excited, especially given how well BG3 has done. I really hope we're entering a new golden age of NWN style CRPG modding fun!