WOW! What a great megathread! I have read every single comment here with interest and everyone here has clever arguments. However I am with the six party side. I am a D&D veteran and played many D&D games like Planescape Torment, Icewind Dale series, Neverwinter Nights series etc (but not only D&D, I generally like other party sized RPGs like ShadowRun series or even the great SpiderWeb Software games) so I have much experience. To play the devil's advocate 4 party sized tend to be good and more balanced and their biggest strength is their replayability. You replay the entire game with different party members -classes so in each playthrough there are different experiences, different dialogues, different choices, different strengths and weaknesses in terms of gameplay, etc. It is an RPG game after all!
However their biggest strength (replayability) to some people is the biggest downside! I personally do NOT like to leave not even a single companion out of the adventure. I want to FULLY experience the plot and all companion's dialogues-interactions between them in a single playthrough. And I personally prefer that ALL companions learn & react about my choices in the adventure... To make matters worse real time limitations, busy schedule + other RPG games are limit factors that force many people not to replay an RPG game. After all the biggest percent of the game (main plot) you have already experienced it, why then spend hours and days again to experience some other NPC party interactions which is just a small % of the game? Plus it just does not feel right plot-wise to leave companions in the camp, especially Origin companions - what are they doing? Taking a nap all the time?? while you go to adventure, saving the world etc. You are a party, and all party members are supposed to be together or else they are not party members, just visitors!
Reading all the discussion in this megathread I understand that Larian will never implement 6 party. Moreover I understand by increasing the party size to 6 the game might be to easy (I dislike RPG games that are easy even in harder difficulties) so I want to report you all some quick research about the many mods available:

First of all some of you might already know the existence of "Party Limit Begone". However this mod is a pseudomod. What I mean is that it indeed increases the party limit in terms of gameplay, but in terms of plot-wise the party stays at 4 members. So in many situations like cutscenes the party is only 4 members, and in all situations like choices and party interactions, only 3 characters closest to the main hero are counted. A user at "Party Limit Begone" post-discussion at NexusMods site at 26/2/2024 named MufasaIllegitimateSon reported that he combined the "Party Limit Begone" with "More Reactive Companions" and with "Everyone In Dialogue" mods in order to limit those gaps. He said that he had enjoyed that combination of mods. However according to current data it does not worth to play with those mods because they are pseudomechanisms (as far as I personally believe). The plot is designed with 4 total characters and by increasing the members some companions might not participate and only appear in the party virtually. It is better wait for complete bug-free versions of those mods or when Baldru's Gate 3 is fully finished (with all patches, potential DLCs etc Do not forget that even the 20 years old Baldur's Gate 1 & 2 are being patched to this day)! So to be frank I never tried these mods, so I report them to search if anyone has some experience with these mods!
If you play with increased party members, then it is mandatory to increase the difficulty by installing those mods below: By some quick search I have found some nice difficulty mods to have are: "Tactician Plus", "Combat Extender", "Lethal AI Enemies", "Immersive AI", "Stronger Bosses and Enemies" which its author recommends other difficulty mods "BG3 Fixed - Fixes and PnP Rules", "Tavern Brawler Rebalanced", "True Initiative (D20 Initiative Rolls)", "Configurable Enemies (Script Extender)", "Enemies Enhanced" which has two versions, the normal "Enemies Enhanced", and the other one is the "Enemies Enhanced (Honour)" combined with "Enemies Enhanced Correction Patch". Another noteworthy difficulty mods are "Additional Enemies - Bugged Clone Fix", "Death March - Enemy and Combat Encounters Overhaul", "Nightmare Difficulty", "Absolute Wrath". Finally, a nice difficulty mode is "The Epics of Tavs" mod which is large and automatically contains other mods like "Party Limit Begone", "Lethal AI Enemies" "Stronger Bosses and Enemies" etc. Moreover there are other gameplay mods that improve the content, but we must make sure that all of those mods I have written are compatible together especially some years later when Baldur’s Gate game and its mods are reaching in their final versions. Such mods are "Extra Feats At Low Levels", "No Inspiration Point Cap", "UnlockLevelCurve - Level 13-20", "5e Spells" and all other spells mods the author of 5e spells reports. "Zerd's Rules as Written (RAW)", "Homebrew Spells", "War Caster 5e", "5e Spells - Larian Durations", "5e Spells References: Valdacil's Spell Adjustments", "Secrets Scrolls for 5e Spells", "Actual Green Flame Blade", "Spell List Combiner", "Minthra's Configurable Enemies")

I know that they are too many, I have never tried them, so I want to learn which combinations of those difficulty mods are the best to be combined with Party Limit Begone. Can all of them be combined? If not which combinations are the best?