For what it's worth, a 5 or 6 character party size will probably be as easy to mod in as it was in D:OS EE, though this may be more problematic and buggy than the first game with the various character interactions that will presumably be in the final game. And of course it will totally break balance, but enemy and party strength can be tweaked somewhat to compensate at least partially.
I wouldn't be too concerned that a well-balanced party will run into some extremely difficult set of fire enemies, that if only you had a water mage would be beatable. Sure, the water mage would be very useful, but not required. Most of the skill trees should be versatile enough that you'll be able to memorize different skillsets for certain fights to accomdate for different tactical approaches.
