Larian has already expanded the UI so that you have room for six character portraits. It would't be dificult to make a six man party available for the players. Those that want a four man party need not take any more, but we who want a party of six would be free to do so.
But there are a couple of marginal problems with it:
- first, there are some occasional "bottlenecks" hardcoded into the game where you either ungroup the extra party members temporarily before doing something (i.e. taking the boat in the Underdark to reach Grymforge) or they get automatically "killed and glitched away".
- second, the DEFAULT formation for the party absolutely fucking blows, as it's this awkward, inconvenient widespread triangle, and I'm not sure if modders will be able to help in this sense: