Here's my pitch for putting Sisko in Star Trek First Contact:
Sisko is in command of the Defiant with Worf at tactical. When they're rescued everything goes exactly the same except there's a lot of tension between Sisko and Picard because Sisko is (understandably) worried that Picard may have some sleeper code buried in him somewhere that'll be activated at an ill-opportune moment. They get a scene where Sisko walks right up to the line of demanding that Picard relinquish command but backs down when the Borg sphere starts to pull it's temporal shenanigans.
Fast forward Sisko joins the landing party and stays behind to help rebuild. A lot of his story beats follow Riker's at this point and he's putting on a genial face, geeking out with Geordi, getting a bit hero-flustered at Zephram, etc but it's all an act, internally, he's fucking fuming that he's not up on the ship taking care of the Borg and at some point Troi calls him out on it. They get a scene about how much the Borg took from him. How much he's grown on the station, how much Bajor means to him, but ultimately how much it's killing him that he can't be up protecting the ship now. She gets to then call him out that it's not about protection for him, is it? You then get one of those very tense Avery Brooks moments where he just looks at her and says "I genuinely don't know, Lt."
Now, one of the big changes this necessitates is that the ground crew needs to know what's going on on the ship, I'm not super sure how to make that happen, but have Geordi technobabble the whatevers and they start picking up com traffic from the ship interspaced with Borg chatter.
Sisko makes a big deal about getting back up to the ship but Riker says they need to trust that Picard has it well in hand, their orders are to make sure the test happens on schedule and the timeline is preserved. Sisko realizes if he puts his foot down Troi would probably rat him out, deem him emotionally compromised, and since this crew is fiercely loyal to Picard, the only thing he'd accomplish is getting himself taken out of command.
Fast forward to the day of the test, Sisko takes Geordi's seat. At one point there needs to be a point where this dialog exchange happens:
Cochrane: Warp engines charged! Initiating warp!
Riker: Standby, the Enterprise is in our way, if we initiate now we'll ram her and spread our atoms across the system! Ben, hit the thrusters and change our trajectory!
And Sisko is within reach of the warp button. If he hits it, sure he'll die but he'll take out the Borg, Picard, and the Vulcans will still see the warp test. The timeline should be fine right?
Ultimately he backs down, changes course and let's go. Trusts Picard.
Give him and Picard a moment in the epilogue where they talk. Lilly told Sisko what happened aboard the Enterprise. Sisko confesses what he could have done. Sisko says he knows it couldn't have been easy for Picard to put aside his hatred like that and doesn't think they'll ever be friends but that he respects the struggle he's gone through, forgives him, and hopes he's forgiven himself. Picard replies that it took courage to tell him all that and that he's been following his career since Sisko took over the station and he can see what a fine captain he's become. And Picard too hopes that someday he's able to fully forgive himself.
