Too be honest I don't see good reasons to change it now. From most players perspective party of 6 is old fashioned and mostly just matter of personal taste. From sales perspective Larian way of doing things works and most people agree with it. Finally the most important from game design perspective everything is written, design, made and balance around party of 4. So chaining it now would created unnecessary chaos to please just the one group of people.
So making party of 6 a basic feature of the game? Big no. Making engine handle party of 6 so mods will allow it without any problems? Yes, go with it.
i agree with your thoughts that at this stage in the game it may be difficult to accomplish, particularly if larian has designed the game as you indicated above around a party of 4 (we sure this is the case?) - but respectfully i disagree and think that making 6 the party size should be the standard as it was the standard in the og bg games and this is supposed to be bg3 (altho reviewing some other posts i feel like some would prefer this as dos2: baldurs gate edition - i dont see enough significant enhancements/improvements to this game that would cause me to even call it dos3). also, please note that by making the standard be 6 slots still allows for players to have a 4 person party (or solo plays) so having a party of 6 isnt alienating a segment of the fan base whereas the reversal seems to be as a card carrying member of the pro6 party. and again, you say that the game is balanced around a party of 4, but larian themselves has said that you can recruit 'mercs' to round out a party and some have been able to get up to 6 in mp so does this argument really hold any weight?
i still struggle to see any concrete points as to why larian wouldnt want to bump the party size up to 6 other than -resources/money (they just crushed their ea sales so i doubt that is the case - if anything it should show them the potential this game has and reinforce larians desire to work with the community to make the game even better than it is currently), -timing (we are still a year out and this is the whole purpose of ea feedback, and larian has released multiple support patches post their dos1/2 launches so i suppose this could be done similarly, but i have my apprehensions), -mods will do it (spending $60 on an ea game where one of the largest feedback points isnt being heard only so mods can support or what many consider fix a primary gameplay mechanic just doesnt sit well with me), -balancing issues (which i dont think has a lot of weight given if you are lucky/save scum with die rolls you can bypass various encounters - i also dont think encounters [outside of random encounters/mobs, which is another game mechanic that larian seems to be moving away from relating to the og bg games] should scale either, as it detracts from a sense of character progression if the world levels with your party but thats a post for another thread)
i would just refer folks to the variety of valid and thought out posts that discusses the rationale and reasoning for having a party of 6 be the standard while still allowing for solo and party of four runs, and challenge ppl to give productive and constructive critiques, around both good and bad aspects of the game, instead of falling back on any of the above tired arguments against parties of 6 or saying that from a sales perspective that larians way of doing things works (debateable) and most ppl agree with it (again, debateable) as this just comes across as excuse making/fanboyism for larian and really isnt productive.