And if Sub-Commander is one step below Commander (which appears to be a standard starship CO rank) in the Romulan system, then her rank would essentially be the same as Sisko's at the beginning of the show - high enough for a fairly commanding position that would probably involve proficiency at multiple fields. Perhaps even too high to be of dramatic use?
A Romulan character would have been a nice addition when the show went Defiant
-centric. Possibly difficult to write for, though, as "The heroes suspect she is scheming behind their backs" gets old pretty fast; Garak already fills that niche anyway; and moving on to a more generic role for T'Rul would just water down the character.
So, IMHO, much better to have her return in S7 or S6 than have her as a semi-regular... But that doesn't say much yet, and any return would have to be heavily justified when the show was already crowded with characters.