Now that you mention it, it would have been a great idea to introduce an unjoined Trill recurring character some time during the show - not in order to replace Jadzia, but in order to explore the Trill society and culture, the relations and differences in status between the joined and the unjoined, etc. This was never really properly explored on screen (the books do a much better job of it), and it provided so many opportunities. Why not have another Trill character? They had all those recurring Ferengis, and that required a lot more makeup, while OTOH, you just need to put a few spots on a Trill actor. Not to mention that Trill society, IMO, would be a lot more interesting to explore than the Ferengi. It would have also been a great way to give Jadzia more to do - and explore her Trillness, instead of making her episodes all about the Klingons, one of the most over-used races in Trek. Ezri could have been portrayed as a Trill who didn't even want to be joined (like she was portrayed in "The Lives of Dax"), which would have provided a different perspective: all the other Trills we ever met were either joined, or desperately wanted to be joined.
And having Ezri as a counselor helping Miles overcome his experience from "Hard Time" would have been also great, as that episode really needed to be followed upon.
And having Ezri as a counselor helping Miles overcome his experience from "Hard Time" would have been also great, as that episode really needed to be followed upon.