If they made a setting at the start of the game as to how many party members you wanted, then all the encounters would be based off of that. If you want a 4 party team, great! The CR and number of creatures you encounter would be based off of that. If you wanted 5 or 6, then the CR scale would slide based off of that.
That might be the ideal design, but a huge problem is that it would require much more play testing and balancing than designing encounters for a single maximum party size.
Each different party size would require either a change in how powerful each enemy is, or how many enemies are in the encounter. And then every permutation would need testing and balancing. That's a lot of work, if the goal is to make each party size have the same amount of challenge. Which is why most games don't do this.
Isn't that why we are here though?
I'd take some level of randomization there over just static encounters that just never change. We already know this can work, since we've seen it in operation in BG1/2.
I think it would be an impetus to address other issues with movement/UI.I don't think space is at such a premium as others do, even in the dungeons. There is plenty of room for gang upon gang of enemies, and allies and randos in the big brawls. There's plenty of room for 2 additional party members in every area I've seen.
They should patch in 6 sooner rather than later.
That would immediately shut the door on all the "this isn't BG but DOS" convos and signal that the devs are willing to at least throw their returning BG fans a bone on something.
I still don't buy the table top analog from the PHB/DMG 3-5. This is computer game, the computer is handling most of the hassle that usually attends to playing a large group session. It doesn't need to just be a simulacrum of the standard campaign, it should be epic. Epic = 6