Adding enemies would just drag out combat in a turn based game. I mentioned an example earlier. They'd have to increase HP a bunch.
If they don't rebalance, 6 will make it too easy. If 6 is optional ill feel compelled to bring them along anyway because "it's obviously intended this way, otherwise they wouldn't have made it this way.". As I'm sure you know, bringing 4 now is optional, as all of EA has been solo cleared, and that's not something I'd do, because it's not intended, just possible.
If that makes sense.
I think you misunderstood me. I agree that adding enemies would drag out combat and is the ~worst way to rebalance for a 6-person-party.
Let's assume that Larian balances for an optional 6-person-party in such a way that combat difficulty and length remain the ~same as for a 4-person party (split exp or increased enemy HP). Would you be perfectly fine with playing a party of 6 in this scenario? Or would you reluctantly play with a 6-person party, feeling compelled, but would have been happier if Larian had stuck with 4?