Q's reference to a Romulan Empress would seem to indicate that the Romulan Empire has some form of monarchy, possibly a ceremonial one, where the empress served a role similar to Kahless. Another possibility, however, might be that the empress may be someone from the Romulan Empire's past, as time traveling is not beyond the abilities of the Q. In the non-canon timeline of Star Trek Online, the position of Empress was reestablished by Donatra in 2381 following a power struggle in the wake of PraetorShinzon's death. Thus indicating that there was an Empress prior to Donatra at some point in Romulan history.
Simon refers to him in the past tense, suggesting he has died, although given the long life span of Romulans, it is unclear whether this happened in the 23rd or 24th centuries.