Friendly newbie here with a newbie perspective! (Obviously, in the end I speak for no one but myself, though c: )
I've read the entire discussion on this thread, and these are my thoughts:
A party of 6 makes sense in many scenarios. Personally, 6 would be too many for me - at least in my current state - but I really don't see why they wouldn't introduce the OPTION for 6 party members. Especially cause this allows you to both take the team setup into consideration (getting a front liner, healer/support, heavy damage, assassins, whatever more one would like) while still leaving room to bring along additional characters that one could want just cause of their personalities. I mean, imagine the following: you really enjoy the roleplaying of a cleric character. But at the same time, you really wanna bring Shadowheart along cause you enjoy her ... Personality. But having 2/4 party members in a group being clerics is... Well, ehm, maybe not optimal. Oooor if you want to play a wizard, but really wanna bring Wyll and/or Gale along cause of their ... Personalities. Having a party restricted to 4 members forces you to choose either functionality or from a roleplaying perspective, ALTERNATIVELY forces you to make your own character fill a specific slot to fit the party (which might not be ideal for many, I know it is not for me!). Obviously, if you wanna bring Wyll, Gale and your own wizard, you're free to do so - but that's not an ideal party setup either, especially not for the less experienced player (such as myself).
In a roleplaying game, I'd argue that, on the easier difficulties, roleplaying should be the first priority. Having a larger party size makes that possible, as you can fit both functionality AND personality-preferences. While also allowing players to properly appreciate the depth of more companion-characters - win-win!
If people are worried about balance (why would you go 4 if you can go 6 for those who prefer 4-man party ) then they could introduce the solution that MrFuji3 came up with: * To have the exp split between party members, causing a party of 6 members to require more exp than a party of fewer members. * Alternatively I thought that they could simply introduce a party buff that increases base stats depending on the number of party members. So, if you run solo = strong buff. If you run with 6 members = no buff / very, very weak buff. * Oooor, they could use the Diablo system (entirely different game, I know - using it as an example) - making enemies just scale up in flat HP and power depending on the amount of party members (within reasonable parameters of course).
Personally (and this is probably not gonna be the meta-opinion here xD) - I prefered the DOS2 system where you could have pretty much any character doing pretty much any role. As in, you got to choose their role when they entered your party for the first time (and then you could change it around as you wished along the way). I get that this is not suitable for BG3 in particular though, and hence I would not even ask Larian to consider introducing this as it is simply not a part of this universe - but it made it much easier to focus on characters of interest without sacrificing the team composition. A bigger party size is an alternative way to handle this dilemma, while staying true to the BG3 universe. c: