If Mako had not died? I think there would have, if you look at the first season there several clues hinting towards a spirit world season Iroh saw Roku's dragon, Zhao mentioned Iroh's trip to the spirit world, and there was a hint that Koh would return.
And if you noticed when Mako died Iroh was barely in the third season and the whole third season wasn't treated too well like the first two.