I agree that adding one or two party members would make the game more fun. As a four person party I often have to choose which class functionality to leave behind to bring one of the NPCs for their quest. example, in my current play-through I'm a wizard, so my main party is me, Lae'zel, Shadowheart, and Astarion. So when either Gale or Wyll need a quest done, (like Wyll wanting to kill the goblin leaders) I basicaly have to pick between Lae'zel or Astarion to leave behind...because ya'know... healer.
A 5 person party would fix this completely and wouldn't require much difficulty tweaking at all.