So I get the want for 6. 6 is typically the maximum that DMs will allow, as it does slow down the game.

However, in a video game I see nothing against it.

Because Larian seems pretty adamant against it, I can see four available options for them:

1. Larian makes parties of 6 available. This allows for more flexibility, and more story party characters (As they already confirmed more will be included).
2. Larian makes parties of 5 available. It allows for the basics, and one additional party member (Or yourself) as the "Whatever I want" character.
3. Larian makes parties of 5 available, and allows one "Follower" companion from camp to join you on adventures. The dog, the goblin, whoever you recruit.
4. Larian sticks to their guns and keeps parties of 4.

I genuinely don't see an issue with parties of 5 or 6; if difficulty is what people are using to argue against it, then why not just throw in an extra mob or two that only spawns in to the world map when you leave camp with that additional party member(s)?
Likewise, you would subtract a mob or two for less characters.
If they implement genuine difficulty settings at the beginning of the game, you would increase mob numbers, rather than flat stat buffs like other games. Flat stat buffs can only get so interesting; extra no-voice no-name mobs makes things more interesting, dynamic, and diverse in combat.