Just because DBZ is popular doesn't mean it's good. Comparing DBZ with Avatar is kind of silly. DBZ is about the battles, the story is merely an excuse to get the cast to fight bigger and stronger characters. In Avatar, the story and character development is way more important than battles.
For me, I'd be disappointed if they made a spin-off. Every story has a beginning and an ending and I think they've reached the ending here. Any spin-off or new powerful enemy would feel like milking the cow. The only spin-off I could see that could expand the story in a positive way would be one or more episodes as Tales of Ba-sing Sei, which would show what characters are doing now, seperately, after the war.
I really liked the ending, a nice mix of storytelling, action and emotions. The cinematic music was great. Azula was a bit of a downer, she always seemed in balance with herself, but now when she freaked out I was surprised. I guess Ty Lee and Mai's betrayal did more to her than we imagined. The lionturtle was weird, they could have had a better character or creature teach Aang to take away bending.