It won't happen. The game's bones/core is essentially done at this point. Larian wouldn't market Early Access where core game features can be altered. They have built this game for a party of four and that's what we are getting.
Very sadly, I have to agree. And it's not just that they have essentially finished core development of the game. It is also Larian's/Swen's "vision" for the game. That vision is something they very firmly believe to be awesome, so why would they change any part of it? I, and some others like me, however, consider that vision to be utterly stupid and ridiculous. But Larian is not going to give us the game we want. They couldn't care less about the game we want. And over time I have degectedly come to accept this reality. The only possible salvation of this game for me is if there exists a modder out there with the skills, time, and willingness to literally take an axe to the game, and remake the game as a true D&D and BG game by rebuilding many of its systems. Then and only then will this game be a halfway decent game for me.
They have actually stated that the UI was possible to extend to a 6 person party out the gate no complaints either way but i wouldnt mind seeing them actually adapt it and it is possible they said that with mods it would be possible to do so regardless. So it isnt exactly impossible to see the game change to seat a 6 person party with multiplayer you can even achieve this in the opening of the game going as high as a 6 person party however limitations in multiplayer prevent you from taking out characters who have joined after the intro. But it is possible to get a party of 6 characters even if it is for a short period so the option does exist as a possibility to do so.