On a side note, while I personally genuinely don't give the tiniest amount of shit about the multiplayer angle* I'm not sure why the most classical party of SIX shouldn't work in multiplayer.
Given that you can ASSIGN characters to the players at will, you could have two players with 3 characters each, two with three, three with two characters... Or any other arbitrary asymmetric combination, really.

There would be nothing intrinsically impractical about it. Well, apart for the game controls, that suck and are impractical by default. More aggravatingly so when you LOWER the number of players involved.

*elaboration: frankly I tried to play previous Larian games in multi and the verdict was always "Well, this was amusing but I'm not going to coordinate a schedule with multiple people over an extended time frame to play like this again".
So no, absolutely not a selling point for me and even less something that I would let DICTATE major design limitations.

Party control in Baldur's Gate 3 is a complete mess that begs to be addressed. SAY NO TO THE TOILET CHAIN