It's a bold decision, if Larian goes that way, no doubt. Still. The more I think about it, the more I like the idea of locking the party in regards to origin characters. Commitment. Feels nice.
That's not to say that I think it's bad to have more party members.
Here's what I'd like to see:
A few of the origin characters stick around. The others run off to do things that need taking care of, and probably come back later in helpful or harmful roles, depending.
Which leaves the main character plus three other origin characters.
On top of that, I'd like the game to allow for more companions, albeit ones without tadpoles. Just regular folk that can be recruited. Followers, cultists, mercenaries, whatever.