"I'll give you a spoiler. Cabbage Merchant. Fire Lord Ozai. Same guy!" Byran Konietzko at SDCC 2007, heard with my own ears! After hearing this, I began to wonder how such a thing could be possible! Well, here's my genius hypothesis.
He looked ordinary enough. He sold fairly ordinary looking merchandise. He traveled with his little cart in a somewhat ordinary fashion. But despite all this outward normality, the cabbage merchant had a terrible, terrible secret.
He was Fire Lord Ozai.
Oh, stop laughing already. It's true!
Really, you can stop laughing.
Fine then! Just keep laughing! I'll keep going.
How can this be? you ask. I won't go into details, but it involves Fire Lord Azulon and his wife, Ilah.
But why would the all-powerful terrifying Fire Lord disguise himself as a cabbage merchant? Well, you're about to find out!
It's the cabbages. Why do you think he cares so much about them? There's a teeny Fire Nation soldier in every cabbage, and Dark magic brings them alive and makes them grow to normal proportions once he's inside a major city, unleashing his cabbage soldiers to take over the world!
Okay, please, enough with the laughing.
Do you not see? He tried to get into Omashu, and his cabbage cart was destroyed! He was incredibly upset, because his cabbage soldiers were inside! Then, he was forced to wait for the Avatar to leave before unleashing his cabbages and taking over Omashu! Who do you think ordered that statue of him built?
Am I starting to get through to you? Are you beginning to see the connections?
Let's move on. The poor cabbage merchant (who is really Ozai) tried to get on the ferry to Ba Sing Se (to take it over with the cabbage soldiers), and once again, his cabbage soldiers were destroyed and he was utterly horrified. That plan was thwarted, so he sent Azula in to do the job, and she did quite nicely. Of course, she was completely unaware of his cabbage soldier plan. Ozai didn't really want to admit that he failed at something.
It is happening? Are you beginning to believe? Are you beginning to question why the cabbage merchant is everywhere the Avatar is?
Trust me, he is Ozai.