If I could magically go back in time and change just one thing about DS9, I'd introduce Ezri to the show a lot earlier, all the way back in season 4. Instead of the never-seen Counsellor Telnorri, we would have Counsellor Tigan, fresh out of the Academy and determined to prove herself. She could be a background character at first, briefly seen in the context of Miles continuing to get therapy (which would also address the issue that the events of Hard Time are literally never mentioned again).
Then we could start seeing more of her as a person. She could be friends with Ziyal. She could maybe have a bit of tension with Jadzia, just to explore the perspective of an unjoined Trill who doesn't think joining is all that great. We could eventually have a whole episode focused around her, introduce the fraught relationships she has with her family even before the joining; it could replace Honor Among Thieves and still tie into the Orion Syndicate thing. And then eventually we would see how the unexpected, unwanted joining changed her. We would see the people in her life react to the change. It would have been great and I'll never not regret we didn't get to see it.