To respond more directly to the OP:
Zuko and Toph would be weird on every level and it would be several years before she could pass the half his age plus seven rule, plus why are you even thinking like this.
Zuko and Katara would've made no damn sense since there's probably literally some PTSD going on there. You can work with the guy whose family nearly wiped out your people and who spent a long time personally trying to kill you I suppose but dating him would be a different thing entirely.
As royalty it's not like Zuko could just marry any random girl he had a crush on anyway....although I don't even think Katara fit even that criteria outside of the fevered imaginings of horny fanfic authors. Out of all the possible characters on the show Mai seemed the most realistic even though in Korra it was said they broke up.
Here are a bunch of people earnestly debating the mystery of whoever it was Zuko wound up marrying. Lots of people seem to think it was Song...I had no idea who that was but it would seem to be the girl he stole the ostrich horse from in Zuko Alone, because they bonded over the scar on her leg you see and therefore were meant to be.
Not that Katara and Aang are all that great a match either but that's the problem when you limit romance options of people with their whole lives ahead of them who will meet other people every day to a dating pool limited forever to just the handful of other major characters they don't happen to be related to.