I've been following this discussion from the beginning, but I have to say that the game would be a mess with 6 characters at the party. For the following reasons:
1. Having to manage 6 inventories with the current system would be a nightmare (and especially with the chain/unchain movement system).
2. 6 party members can work well in Real-Time with Pause, where you can let some of them just do what the AI tells them to do, but in Turn-Based Combat the fights would last for hours, damaging the pacing of the game.
3. Larian has expertise in making 4-character games. Asking them to change and manage it now can irretrievably damage the balance of the entire game (which is not yet perfect).
I know the current 4-character system is quite limited and makes experiments more difficult (why would anyone stop using the same tank-rogue-mage-cleric party?), but this can be improved in other ways. In some quests, we could have an extra fifth character, linked to the quest, or have to obligatorily use a different character to try out other ways of playing. I like how in PoE (first) we occasionally need to send a character on a quest, and that makes us have to put another party member in his/her place.
Lastly, maybe I say this more because in my years playing table-top RPG I rarely played with more than 4 people, and when that happened the game was always a mess. I particularly believe the game has a lot to lose if you place a group of 6.
