actually, zuko being either lin or suyin's father makes the most sense?
especially lin's bc it doesn't make sense for toph to refuse disclosing her father's identity at the cost of staining their relationship if it was anyone else.
toph needed her freedom and wanted to be the republic city's chief of police while zuko was the fire lord and his family would have to follow rules. i bet toph didn't want lin to be confined in the palace the way she used to be as a child either.
revealing that lin was zuko's would complicate things for both of them, so she kept the truth to herself and then it was too late.
i know she mentions the father's name being "kanto," but i think it might be a code name they used for zuko at some point? bc again, doesn't make sense for her to hide his identity if he was a regular guy.
this theory lowkey works for suyin too, tbh. or it could be that suyin is sokka's and toph just didn't want to overcomplicate things by telling suyin who her father was and not lin. or if she said anything, then it would be unfair for sokka to know he had a daughter and not zuko. and if they all knew everything, then it would just not be comfortable.