Too be honest I don't see good reasons to change it now. From most players perspective party of 6 is old fashioned and mostly just matter of personal taste. From sales perspective Larian way of doing things works and most people agree with it.
Finally the most important from game design perspective everything is written, design (gameplay, multiplayer and so on), made and balance around party of 4. So chaining it now would created unnecessary chaos to please just the one group of people.

So making party of 6 a basic feature of the game? Big no. Making engine handle party of 6 so mods will allow it without any problems? Yes, go with it.