A/N: I know that people like Comodin Cam have made videos covering this kind of topic in regards to Optimus Prime getting a solo film in the continuity of the Bumblebee movie.
To start, I think the beginning of the film could leave off where The Cybertron Scene in the Bumblebee Movie left off.
Optimus was surrounded by Soundwave, Starscream, Shockwave, Shatter, Dropkick and a whole bunch of Seekers and he was still alive by the end of the movie. The film could give a interesting way of showing how he managed to find a way to escape all of them and leave Cybertron. Maybe he got help from one of the Autobot forces who probably choose to stay behind like Elita-One.
The story of the film could focus on Optimus having to search for his fellow Autobots who left Cybertron and are on other worlds. He could try to find them and inform them on the planet for them all to meet up on. In the search, we could see who all of the 7 Autobots who were landing on earth. (We know it's like Ratchet, Ironhide, Wheeljack, Brawn and Arcee. But we don't know who the other two are). Maybe the other two could be Jazz and Prowl.
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During his search for the other Autobots, Optimus would also have to find himself being hunted by this universe's version of The Decepticon Bounty Hunter, Lockdown. Lockdown could be hired by someone like Starscream, Soundwave or even Megatron. And Lockdown could have a band of Seekers with him to give Optimus more Trouble (Said seekers could be memorable ones like the cone-headed one or even some like the rain makers).
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(And in this film series, Lockdown could actually be part of the Decepticons, albeit a rouge).
Another interesting thing that the film could do is show flashbacks to the days of Cybertron, before and during the War. Giving us an onscreen look at a Pre-War Cybertron and the film could give us our first look at the universe's version of Megatron.
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And even in the flashbacks, we can see how the relationship between Optimus and Megatron was before the two eventually went to War with one another and possibly why. Maybe something like Megatron being denied something that he felt he deserved but Optimus got it. Or It could be something with Megatron going against a corrupt Cybertronian Government and his actions (such as Megatron killing a previous Autobot leader) cause Optimus to step up and try to get him to stop. Which could lead to a lot of Emotional moments.
The film could either end with Optimus and Lockdown facing off one on one on some sort of ship that Optimus could use to travel to Earth and either Optimus could have no other choice than to kill him or Lockdown manages to escape and survive. Optimus could then take the ship, go to earth and meet up with Bumblebee like he did in the Bumblebee movie.
An after credit scene or final credit scene could be that the Decepticons figured out where Optimus and The Autobots are currently at and they now set their course for earth, meaning that the war will continue on earth.
