I've played with a six member party with a mod. It's not cumbersome at all. It does obviously trivialize the current encounters even more though. That means, Larian would have to rebalance everything again.
It won't happen. The game's bones/core is essentially done at this point. Larian wouldn't market Early Access where core game features can be altered. They have built this game for a party of four and that's what we are getting.