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.
I have nothing to support this, but I wonder if there was some discussion about writing Garak off the show in lieu of T'Rul. I can't imagine why that would be, but if DS9's producers ever thought about using Martha Hackett for more episodes, Garak would seem to be the odd man out.
OTOH, maybe T'Rul could have played the Eddington role and eventually joined the Maquis, betraying both Starfleet and her own people to join the terrorist group. Even knowing about the Romulans' enmity toward most humans, it's not hard to picture her empathizing with Federation colonists who feel like they've been left to fend for themselves against the Cardassians.
If nothing else, it would have been a riot to see how she got along with Worf after he joined the cast. To have a Klingon, a Cardassian, and
a Romulan aboard DS9 would have been interesting.