May be obvious, but "It will be just like the good old days," still got me. Azula's finale had some tragic bits as well surprisingly. I thought her mental breakdown was the most engaging thing we saw for her character.
Iroh would have gone on being all badass war hero, destroyer of the earth kingdom. Never would've doubted the rightness of warfare. Probably would've gone on to take out the water tribes back in the day, too. With the water tribes gone, no Sokka/Katara to find a frozen Aang. With the fire benders everywhere, Aang probably would've thawed out sooner, but would be all alone against the fire lord, with no one to teach him water or earth bending.