It's not that we don't have enough information. The fact that there are long-running megathreads going back for years without any attempt at implementation is evidence that these things are not being implemented (or are much lower priority).
Edit: Also, if Larian were considering rolling out a larger party, given how it would fundamentally change combat, they would likely want to test it on early access. The lack of the presence of something that would fundamentally alter combat at higher difficulties in the early access months before release date is evidence to non-implementation. Larian wanted to test reactions, so they put them in early access. Same with flight. And classes. But not a change in party size, even though that would fundamentally change the game balancing?
I believe that you are making several assumptions here. Lack of testing in the wider open beta is not proof of anything other than that it isn't being tested in the wider open beta. Not that it isn't being worked on. And as for the impact on balance, there is a whole lot of data that can be extrapolated based on the gameplay as it stands, which could help with balancing issues for this type of change. As example, the entire 2nd and 3rd act are not being balance tested. Because they are using the balancing for act 1 to tweak it.
I am not saying that they are (or are not) working on a larger party. I simply do not know. Just pointing out that the assumptions you lay out above are not proof one way or the other either. I agree that they are compelling, and point to them not doing it. But that's an assumption.
Anyway, the thread was "what would you like to be surprised by". If they did that, and I am not saying they are, I would like to be surprised by that. I make no predictions, merely that it would be a nice surprise IF they did that. Well, it would be a nice surprise to me.
