I doubt it would require that much effort from Larian to adjust the game engine to handle 6 character groups. If a modder with no access to the source code can enable it for D:OS2 then I'm sure Larian can manage it for BG3 as well.
Not sure what they have planned for the story though and how having a 6 character party would affect that. But hopefully we will get some clarification from them soon. This is a popular topic after all.