I wonder if its possible to have the game auto-balance to 6 party size - or if they would have to go back and re-balance all the encounters for the larger group.

Like the Gith fight, you would need to add 3 more Githyanki for a 6 person party to account for synergy.

OR they could make that the easy mode, where if you select Easy or Story Mode you get a 6 person party. Would that be a fair solution? Upside, no extra dev time needed.