It would be fun if your character get imprisoned by a foe and you have to play as a companion to rescue him/her. You have to choose with which companion to play the rescue mission.
Yep, good idea, but already suggested.
Other players also suggested other companion related things like:
-Being able to form an instance group with your companions (i.e. You + 3 companions) for particular personal quests
-More "real" companion quests, not only the fading screen + companion coming back.
-Including your companions in your legacy
-A better companion AI user interface (e.g. the Tactics interface in Dragon Age, the herald/skirmish soldier/pet bar in LoTRO)
-Companion dual specs (e.g. an AoE spec vs a Single Target spec)
-Companion color matching gear option like for our character + hiding companion headgear