Personally I think it would have been nice to see Ezri a few seasons earlier than the last one. If anything, to have her come aboard periodically to handle O'Brien's horribly handled imprisonment follow-up from the events of Hard Time
O'Brien hates counselors. Ezri's not a terribly good assistant counselor visiting from the Destiny
. But for some reason he takes a liking to her and chooses her for his monthly sessions. Maybe this storyline is just a B-story in a one-off episode but I would have liked to have seen at least that.
It would also be interesting to see an 'everyday' non-joined Trill, who grew up in a mining colony off-world and was never caught up in the idea of becoming joined - and watch how she would interact with a top-of-her-game character like Jadzia. THEN to see her take on the mantle of the Dax symbiont, see the change in her and understand a little better why she's so messed up in season 7.
Obviously the writers didn't have the foresight to predict Farrell's exit, but it would have been nice.