Should there be a sequel/spin off? No. Aang and Zuko's story is complete. Should there be something with new characters? Maybe. I don't know how Mike and Bryan could make a new series any better than the original. Would it be a good idea to have some continuation/closure on the post-war Avatar story? Most definitely yes. The series has an incredibly full basin of rich and deep background story and beautiful characters. It would be a shame if nothing else was done with them.
My 2 cents for the time being.