It's going to be prohibitively difficult for modders to make content that matches the polish of this game's presentation, so I'd definitely prefer if they invested in professional funded teams.
"Same Engine Sequel" is a bit misleading as a colloquial term, and standalone expansion is probably the better one. Like both Fallout New Vegas and Fallout 4 are being made in versions of the same engine, but only New Vegas is a "same engine sequel" because it doesn't have a massive overhaul of the graphics and gameplay and just builds a new game around the same core assets and mechanics.
I was thinking Icewind Dale would be real easy to do. You could get away with not writing any bespoke companions and probably get it finished a lot quicker.