Zuko joining the group, yeah thats definatly going to happen at this point. The only real questions are when, where, and will Iroh go with him.

I have a feeling Iroh will not be traveling with the gaang for one reason or another. Having Iroh there would just throw off the whole group dynamic thing. Having this one old guy traveling around with a group of kids just wouldn't seem right. Besides, if he was there the group together would just be to powerful. Not only would they all be powerful benders, but then they would also have Iroh's wisdom and connections.