I'm completely against Zuko joining Aang's crew...I think Iroh's speech has jarred him in his quest to capture the Avatar, but I'm almost positive something will force Zuko to return to the Fire Nation:
Maybe??Zuko realizes that his destiny is to end the war, he's lived amongst the refugees and knows that the Fire Nation is hurting too many lives, so he sets off on his own to join in the war effort against the??Fire Nation in hopes of getting all the way back to the capital, but leaves Iroh behind, scared and desperate for Zuko...??Iroh seeks out the Avatar, to bring back Zuko...just a wild theory...I wouldn't be at all bothered if Iroh joins Aang, it would still have Aang and Zuko with seperate stories(individually anyway) and Aang would have his Firebending teacher...