you can write something about zuko finding out what mai said at boiling rock/ty lee or mai telling him!
i have a few versions in my mind of how this might happen (each one includes ty lee cause only she has been there when mai said it to azula).
but let’s leave their dating period, pre-breakup, after-breakup, the reunion etc. because now i want to talk about that particular version where maiko are married.
so, mai and zuko are happily married for years now (maybe they have izumi/maybe still have not, as you wish), run the country together and raise at least their one kid – druk.
one day (that can be an anniversary of ending the war and therefore a huge national holiday in the fire nation) the whole gaang got together (+ ty lee). the evening after the formal celebration/feast they all hang out in the palace, remembering the most significant and cool moments from their common past during the war.
of course, sokka retells for the 100th time about their crazy escape from the boiling rock, and then ty lee chirps "that was so cool! when mai said to azula ‘i love zuko more than i fear you!’"
zuko, who’s hugging his wife, stiffens, and mai’s heart drops to the stomach, but their friends haven’t noticed the tension, only laugh and applaud, and go with another story – how katara beat pakku or smth like that.
much later that night when they finally stayed alone, zuko cautiously asks “you…you really said that to azula?”
mai sighs, but replies reluctantly “do you think ty lee is a liar?”
and then zuko is just… silent. for a moment, two, three. but his eyes are glowing, and the expression on his face is so tender (mai only once saw that special expression – at their wedding.)
she has no time to ask what’s wrong when zuko pulls her in a crushing embrace, and then, after a short comfortable pause, he murmurs in her hair “thank you”.
(that's it. thanks for the ask and i'm sorry that took so long!!)