Someone noted in DOS II they didn't feel compelled to add the other 2 characters to their party for another play through just to see their stories and I didn't really either. What WAS fun though was using a mod to have all of them in my party at once so I could do all of their stories and scale up the monsters Which wasn't always very easy and would have been cool to have that built into the game from the start as an option.
I think we can all safely bet that there will be a mod that remove the 4-man party restriction. Whats more of an issue is the scaling of combat, as you point out. Ideally there would be scaling within the game that factors in party size and party level (just like in pnp). If you create a mod that allow you to have 6 members but dont scale the encounters it will suck and its alot to ask from a modder to re-balance every encounters in the game.