Nurse Ogawa would have been my first pick, and most likely the most realistic as she is kind of like the O'Brien of Sickbay: a very noteworthy character and sometimes central to the action, but not part of the principal cast.
My other choice would have been for either Odo or Quark, but DS9 was running at the time. Narratively, it wouldn't have made sense for them to join Voyager unless Odo was in pursuit of a fugitive from the Marquis and assumed he would be sent back to DS9 once the person was captured.
Quark makes even less sense unless he follows Harry onto Voyager to further persuade his transaction, only for Voyager to disembark with Quark on board.
My other choice would have been for either Odo or Quark, but DS9 was running at the time. Narratively, it wouldn't have made sense for them to join Voyager unless Odo was in pursuit of a fugitive from the Marquis and assumed he would be sent back to DS9 once the person was captured.
Quark makes even less sense unless he follows Harry onto Voyager to further persuade his transaction, only for Voyager to disembark with Quark on board.