I think this movie can be saved with plot tweaks and tying up loose ends from the show :

Movie begins with Kira FINALLY breaking away from the skinwalkers, who did teach her control of the fox but delved into some dark, evil things. She finds out they’ve been taking the shape of dead people and attempting to resurrect the nogitsune thinking if they can control Kira’s fox, the nogitsune can be their own spirit they use for personal gain for the failing skinwalker colony. Kira breaks free and warns Scott, who calls Liam and his fiancee Hikaru who are guarding the firefly. They get the message too late however and the nogitsune is freed and the skinwalkers take the firefly to do a ritual on it, that allows the spirit to take the form of one it has devoured. Allison. Kira breaks the news to Scott, Liam & Hikaru, they ask what they can do to free Allison’s body from the Nogitsune’s hold. Kira says there’s a ritual but they’ll need help, cue Lydia, Malia, Jackson & Parrish entering. Lydia explains Stiles is at home with their kids because she made him stay far away from the nogistune and promise he’ll be there for the kids, she also explains that stiles did protest because they don’t want a repeat of Derek & Braeden so she says she promised she’d help free Allison & be back immediately but she wanted him safe and far away from the nogitsune as Stiles still sometimes has nightmares about it. We find out what she means by Derek & Braeden when Scott has to convince Derek to help when he initially didn’t want to because he’s all Eli has after Braeden passed on a mission and it really messed Eli up losing his mother. Derek agrees to help, and leaves Eli with Sherrif stilinski & Mason. Noah & Mason are called in when people start turning up dead due to arrow wounds, knowing it’s Allison, they gang start doing what they can to get her body back, Eli takes advantage of everyone being distracted, steals the jeep and comes to help but falls into the Nogitsune’s trap, and is abducted. Derek, angry that his son’s been taken goes off alone to try and save him but is intercepted by Chris Argent who says let’s do it together. The three of them fight off the skin walkers to save Eli, Eli shifts for the first time and they seem to be winning. But Alison/The Nogitsune shows up with Mason & Noah hostage. Derek agrees not to kill Allison cuz there’s still a chance she’s still in there but holding back almost costs Derek his life, but he pulls through becoming a true alpha. Back with the rest of the cast, they’re done getting all the things they need and with the help of Deaton they’re ready to do the ritual at the nemeton. It’s successful but the Nogitsune only laughs once he’s ejected from Alison’s body, MULTIPLIES and carries the face of that random teacher from season 3 that they brought back in canon for some reason, the skinwalkers, MERGE with the multiples of the Nogitsune becoming an even stronger army. Then they vanish. Weeks pass and everyone’s on edge because they have no idea where the the super Nogitsune army is, but they’ve mostly gone back to normal with Allison back and Eli’s championship lacrosse game coming up everyone’s just gonna go to the stadium and cheer him on trying to have a slice of happiness again when Peter appears with the news that he’s been tracking the Nogitsune army as requested and knows that they’re planning to march on the game. And so, the final battle commences at beacon hills high, Derek dies at the end like in the real movie and is adopted by Scott & Allison and he has his little rawr at the end setting up his spin off.

Clarification edit :

Argent is the one to save Eli and Derek promises to save his child in return, in the end battle, Lydia does her iconic “Allison” scream again but this time Derek pushes her out the way and saves her sacrificing himself to save Argent’s child like he said he would. Has tearful final words with Eli before he passes and they save a second wind and win for Derek

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Kira tries to warn the skin walkers her fox and the nogitsune are not the same thing, it cannot be tamed and will trick them to become more powerful, that’s exactly what happens when the nogitsune merges his power with the skin walkers to create his super army, basically double crosses and takes control of them instead and then tries to destroy the pack.