Functionally, it is all perfectly possible and Larian are definitely capable of enabling fan-created content and PW, if they think there is a call for it.
Possible issues?
1- NWN/NWN2 were built from the ground up as tools to build adventures, BG3 is being built as a one-shot MP-oriented adventure. Larian talk of the engine as a collection of interlocking systems, which sounds good, but may not be ideal for scripting other adventures.
2- Unknown whether middleware is used and whether licence would cover anything other than base game ( NWN2 had an issue with RAD Game Tools "Granny" animation system; there was a licence argument ).
3- Unknown how difficult building additional content would be for this engine, and therefore what the take-up would be.
But, there are teams of people, small and large, that have poured astonishing amounts of time into building their own visions when given the tools to do so, even if their output has limited audience, or none at all.
Of particular note, most infinity engine games have been re-authored using the NWN2 toolset, and both Morrowind and Oblivion re-authored in the Skyrim toolset. But, there are thousands of less ambitious projects producing smaller or larger amounts of content, not just the mega-projects.
