I don't think it would've been particularly hard to plot. Set in the late 24th Century. A composite crew of 7 or 8 brought together from TNG/DS9/VOY. Admiral Picard tasked with getting whoever in Starfleet is available, suitable to the mission, and fill the positions available. Mix it up, without any requirement for it to be any single entire series cast. Parachute in those who've no experience with each other and make them work together. O'Brien never appeared on the big screen, so I'd take him away from teaching to handle Engineering. Seven, now a Science Officer, probably. Flip a coin for Security. Tuvok gets it. Worf shows up Captaining a Klingon ship instead. Data was toast, but nothing preventing a brief B-4 appearance.
Assemble them onboard the Enterprise-E. Cameos for those outside that core of characters thrown together, as long as they drive the story forward. A stopover at Deep Space Nine to pick up a dignitaries. So the station appears, if not whoever's there now. Spock still on Romulus, trying to prevent new hostilities posed by a current crisis. There's a flashback to something the cast of ENT caused. Historic records found, the antagonists are hoping will split the Federation apart. Probably faked evidence, refuted by an eye-witness. Have a 200 year old Shran who show up at the story's climax.
As I say, not hard to plot a story that takes in different iterations of Star Trek and be plausible. Getting anybody who isn't already familiar with this universe to care, somewhat harder... Easier a decade ago, when it was still a going concern on TV. Turned it into a special to celebrate the 40th Anniversary. Nemesis' poor reception put pay to such a thing happening at the cinema.