The only named candidates that I am aware of, based on those Zuko was romantically involved with during the events of Avatar: The Last Airbender, would be Mai, who was Zuko's girlfriend in the 3rd season (Book 3: Fire), or possibly Jin, who Zuko went on a date with under the false name "Lee" during the 2nd season (Book 2: Earth). Of course, the answer could also be neither of these two.
There has been speculation that, because Izumi's name translates to "fountain", this could reference either of the aforementioned two women, since Jin's date with "Lee" ended at a fountain, and it is revealed in the The Lost Adventures comic that a fountain was significant in Zuko and Mai's past. However, since this is all just speculation and theories, these are not at all credible.
Is there any canonical answer to who Fire Lord Izumi's mother was? Word of God answers are acceptable, but speculation or theories are not.