Also, I think it might be interesting to have a checkbox to return to Larian's original idea of making you commit to particular companions at the end of act 1 (and again at the end of act 2). Yes, losing some companions might restrict how the story goes -- but, hey, that's life!

(I'd go so far as to include the "hirelings" in this -- or even make you choose at the beginning of the game if you want hirelings or companions -- and if you choose hirelings, ditch the "vessels" schtick and just let you create a full custom party.)