I hope everyone that advocates for an increased party size also understands the implications on basically every single aspect of the entire game regarding balance.
This would in-effect basically cake-walk significant majority, if not the entire game.
Go play DOS2 with the standard 4 characters on the hardest difficulty (Tactician) then get a mod that adds only even ONE extra character and realise how trivial the game instantly becomes.
There's something to be said for challenge and less-is-more. If they've made/making the game around a party of 4, something like two extra characters, on top of being hugely imbalanced to the entire game, also becomes a massive micro-management of the party. Not to mention the various compounded-effects on the dialogue and story.
Going from 4 to 6 is where you get to that point when you keep folding a paper to the point you can't fold it any more. I honestly just cannot see this happening. As much as people want this to be DND, it's just not.
There are so many variables with even just a party of 4, basically adding 2 more you'd effectively be quadrupling the work required in the game to facilitate this. Entire re-balancing, entire interactions and a whole complex myriad of sequences, dialogue, events, triggers.
I have a belief that they would have come to a decision of 4 by purpose, by choice and with good reason. We may not know it, and maybe it's not set in stone, but i'm almost certain the discussion came up of Party-size at some point and i guarantee you the size of 4 was selected for a lot of reasons. To go back on that, i just can't see it happening. Not this far along...