Ain’t gonna happen - the cost to implement this with the production value the game has.....
It's ain't gonna happen, but there isn't additional cost to having 6 unit party opposed to 4 (especially as people don't seem to be asking for 6 player coop). It would take different UI, if current doesn't support it, but it is not inherently more expensive to have 6 party RPG then 4.
It'll take manhours for rebalancing and other things. So there is a very real cost to it. And we know game devs get paid crap like everyone else, but they still get paid.