Increasing the default party size is not such a simple matter. I've done one playthrough with a party of 6 on Patch 6, and with this many characters you can steamroll almost every encounter using basic attacks and little else. For a proper party of 6, the entire game would have to be rebalanced, most if not all encounters remade.

Don't get me wrong, I'd like a bigger party as well, if only to see more story, but it's not as simple as "just making it official".