I think a more interesting dynamic is already in place. If you choose to do certain things in the game, you can lose Astarion. Other things, you can lose Wyll. I believe Shadowheart will disappear too if you make certain decsions.
This would force players to act in certain ways just to keep a preferred class in their party. If I really want a wizard in my group and there isn't a way to hire a mercenary for that role, then I'm basically forced to keep Gale happy. Or at least not mad enough to leave.