T'Rul would've been a great recurring character, especially after Worf came onboard as a source of animosity, which could've evolved into a grudging respect.
Ro coming in for a guest appearance would've been great, she sneaks onboard, disguised as a Militia crewmember but is noticed by O'Brien. She's on a mission from the Maquis, looking for medical supplies when a plague starts sweeping through the DMZ colonies. O'Brien and Bashir go with her and together they discover that the plague was engineered by the Cardassians, in an attempt to claim the region for themselves. Would've been ideal for a two-parter.
Making more of Eddington would've made his betrayal all the more impactful, though it was still pretty shocking the first time I watched the episode, and even after he joined the Maquis--where they could've partnered him up with Cal Hudson to use Sisko's expertise to take on the Dominion.
Also having Tom Riker return in Season 7 for an episode, rescued by Kira only to be killed in action taking out a major target--could add some interesting tension for Kira, given her relationship with Odo at the time and the chemistry between her and Riker.
Reading your replies, just had this crazy thought. I hadn't considered Worf and T'Rul interacting, but now I've wondered if it would've been more interesting to see a Worf/T'Rul romance than a Worf/Jadzia one. While I was okay with the Worf/Jadzia romance I could've done without it as well. I also didn't like that Troi never showed up on DS9 so we could've gotten some resolution to the Worf/Troi romance-which I liked better than Worf/Jadzia. Also I wish when Alexander was having his troubles on the show, if he could've at least talked with Troi via subspace or something. It's unfortunate they never found a way to work Troi onto the show. I also wish that for the Worf/Jadzia wedding episode that some of the ENT-D/E crew had showed up for that. We know that Burton, Sirtis, Frakes, and Schultz at least were amenable to doing VOY and some ENT, so I assume they would've done DS9, or in Frakes's case, another DS9 episode. A great missed opportunity there.
Unlike you I wasn't that shocked about Eddington's betrayal because he was such a peripheral character that it didn't do much for me. When he first appeared he seemed kind of smarmy and I was hoping we would get more of that and him being a jerk or something, but then he just became a background player. He wasn't given enough development or enough screen time IMO for his betrayal to matter. Perhaps if Eddington had also been a survivor from the Saratoga it would've helped solidify the bond between him and Sisko, though that ran the risk of turning him into another Cal Hudson. But since they didn't revisit Hudson I wouldn't have minded so much.
I like the idea of Tom Riker returning, though I would've been fine if it had been before Season 7, and if he survived. Love your idea for Ro's return. That might have to become a United Trek story.
Another character (s) I would have liked to have seen return are the Borg. I wanted to see Sisko vs. the Borg again. When the movie First Contact was out, I often thought that Picard's words and actions fit Sisko more easily than Picard IMO. Sisko in First Contact would've been pretty epic.
I also would've liked to have seen Guinan on DS9, it was something they had tried to do but couldn't pull off. But also Nichelle Nichols, with a return of Uhura. Granted DS9 brought the TOS Klingons into the 24th century, but Uhura, Chekov, or Sulu being around would've been nice to see.