Personally, I would like the option to create 6 custom party members. It would be nice to give us that option for those who wants it (dont want it? Dont use it! :P ). Im not asking to adjust or change the whole game, just give a warning that the game is intended for 4.
Mods? I know that theres mods for up to 8 party members but I dont think theres any that lets you have 6 custom party members yet. Anyways it would be nice to be able to do that without mods.
I would also be happy about a custom party of 5. That wouldn't be too much to ask, a very good compromise, so to speak, and for that number there are better formations.
EDIT: But to be honest. When you appreciate the Baldur's Gate series, you get one of its core elements par excellence, namely the possibility of a (custom) party size of 6. A party size of 4 = DOS & DOS ≠ Baldur's Gate. As simple as that.