I have written my own versions of all the films, and I figured I would give my own versions of the 4th, 5th, and potential 6th. I will preface it by saying in my versions, Jazz is still alive and Hound has been active since Transformers. Skids kept the same body style just with the head from Canopy and is blue and took on a Kup type role, voiced by the Grandpa from Generator Rex, with no Mudflap. However there were brothers in RotF, Sunstreaker and Sideswipe. Jetfires spark was salvaged off screen by Ratchet, and him and Wheeljack are able to revive him in a new, smaller body, essentially transplanting him into a hatchling. He is now roughly Bumblebee size and young and healthy but as old and cranky in personality as he always was, transforming into a Toyota 86. Sentinel Prime would be Alpha Trion in disguise and wouldn't have been a true prime and his deal was with Shockwave and not Megatron. Soundwave and his bevy of minions didnt die but are taking refuge in the Decepticon city, taking charge of the city and transforming into a Lexus LFA(2011) and then a Lexus LC 500(2017).

First, I would have flipped the names.

Transformers: The Last Knight would be the 4th now.

Then basic plot would be the same, except it would be a much smaller story focusing mainly on Optimus and Lockdown. The Ratchet death would still be part of it for brevity, as well as Sideswipe and Sunstreaker being hunted with Sunstreaker killed and Sideswipe heavily damaged. Cemetery Wind would be shown hunting Decepticons. The Dinobots and get rich quick scheme with the seed would be dropped, as well as the Galvatron plot. Hating the Transformers regardless of faction would be the primary reason for working with Lockdown. Prowl would be a dissolutioned former Autobot working for Lockdown. Wahlberg and his family would still be present. Smokescreen would be present, being one of the few transformers of either side to come to Earth post Chicago. A subplot would be Cemetery Wind hunting him before linking up with Optimus at the end. Optimus would stay in his battered, damaged form the entirety which is how Lockdown is able to go toe to toe with a Prime. The climax would see Prime facing Lockdown on the top of the ship while Drift fights Prowl and tries to bring him back to their side while Crosshairs and Smokescreen fight off the waves of Steel Jaws and Shadow Raiders and protect their new human family. There is no overarching world ending threat, just a cat and mouse between a damaged Optimus, his few allies, and Lockdown. Lockdowns mission was to find the last Prime like in the actual movie, for an unspecified reason. Optimus throws Lockdown off the ship(altho doesn't kill him) and takes it into space.
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Transformers: Age of Extinction
Changes are the Quintessa plot is dropped and Unicron, for now. Optimus is absent, as his story in space is taking place at the same time as the events of the story. Canopy and Squeeks are absent. Izzy is there but is Sam and Mikaela daughter. Cogman's model is used but as a Decepticon pretender, Thunderwing, a second main villain. The quest for Dragonstorm, renamed Superion, is a main part of the story. Infernocus is made up of 5 identical twins who have a bigger role, renamed Abominus, who are scientists and jokesters(think the brothers from Me, Myself, and Irene) but become a snarling monster when combined.
Begins with the same exposition about Transformers laws being passed and more and more coming to Earth. But the film actually starts on another planet, with a race of blue skinned humanoids fleeing an unseen terror and his minions, while a group of humanoids try to fight back. A group makes its way with a data disk but one is killed by a tendril that comes off screen before slipping a disc to his compatriot who is able to mostly escape a swarm of those same tendrils in a ship. The ship is knicked and damaged by a tendril just as it activates a space bridge.

On earth, the Autobots are scattered and in hiding. Decepticons have their own city in the Desert, with anti aircraft cannons and missile launchers set to fire on any movement past the perimeter.

In the sky, the ship seen on the alien world pops up on radar after coming out of the space bridge and is instantly fired upon, crashing in Chicago. As humans set on it with the bipeds seen in TLK and small/large arms, they are saved by Lockdown and Cade. We find out Cade salvaged and repaired the damaged Lockdown, but also planted one of Lockdowns grenades in Lockdowns head with a trigger should he need to use it(basically the scene where Bumblebee leaps thru the building and kills the biped is replaced with Lockdown). Lockdown knows the blue skinned alien(in my mind played by Charlie Day) and explains why him being on Earth is a bad thing. His ship also reveals itself as a Transformer before dying.

Humans and Transformers have an uneasy peace. Autobots and Decepticons don't have a kill on sight order so long as they don't engage in fights. Autobots get to live their lives, preferably staying in their alt modes, Decepticons go to their new home in the desert. Some Autobots, in this case Jazz, Drift, and Prowl help the humans directly albeit with very strict rules of engagement, working with Marissa Fairborne(played by Olivia Thrilby). However, some Autobots live outside the law. Bumblebee in particular leads a group(him, Hound, Crosshairs, Cliffjumper, Mirage, Smokescreen, and Hot Rod), who keep an eye on particularly dangerous Decepticons but only engage when they have to, but the main problem is the Wreckers, who actively hunt and kill Decepticons wherever they find them. The Witwicky family has moved to England under assumed names, with Ron and Judy taking care of Izzy with Skids as a protector while Sam and Mikaela are off the grid with Bumblebee. Humans have developed a new technology that keeps Transformers from transforming, developed from studying the biology. An early scene would show a damaged Sideswipe hiding in a barn but instead of fighting like his first instinct, after he hears them talking about zeroing in on Sam, he scans one of the vehicles and sneaks aboard. At a small village in Cornwall, the humans, led by a character played by John Glover, walk up to a cottage. They place an inhibitor on the Lotus out front. Inside are Ron, Judy, and Izzy who have new names and very poor imitations of Cornish accents. They say they want Sam, he is a fugitive, and threatens Skids, who cannot transform. However, as they prepare to kill Skids, Sideswipe reveals himself and they are able to escape after a chase, with Skids leading them to Edmund Burton, who has set up a safe haven for Autobots, including Bumblebee, with a network of human operatives. Jazz, Prowl, Drift, and Marissa along with Lennox and Epps, are also part of the group. The Wreckers are not acting alone. An alert goes off with the TRF being in hot pursuit of Cade, Shane, Tessa, and a spastic blue alien in dark green Rolls Royce Wraith. Sam and Bumblebee along with Jetfire are dispatched to deal with it, and link up with their covert team in the process. With the group all together, the alien tells the group what is coming and how everything that has happened up to that point is tied together. Earth is Unicron and Liege Maximo is one of the last remaining living Primes. The Primes exist for one reason, killing Unicron. The reason the Fallen irrationally hated Earth and wanted to destroy it was because subconsciously he could sense Unicron. The reason the All Spark and A3s ship were drawn to Earth was because of Unicron. The reason Transformers are raining from the sky is Unicron. The only reason Optimus didn't have the desire to destroy Earth is because it was overpowered by his compassion for humanity and friendship with Sam. Leige destroyed his planet as he had many others as his species were guardians of the secret of Unicrons location. Edmund and Hot Rod then introduce Vivian, who is aware of her connection to the story, her career being a front to search for Cybertronian artifacts and keep them in safe hands. She tells them without Optimus their chances in this fight are limited. But they have a limited shot, an Autobot ship was lost eons ago, carrying 5 knights whose primary purpose was keeping a watchful eye on Unicron. There is also legends of a weapon that can only be activated by a select few but has been lost to history. A subplot takes place in the Decepticon city, and we learn that much like Jetfire, Megatron and Starscream were salvaged and rebuilt, fixed by the Abominus brothers, Megatron taking the design we see in TLK and Starscream being transplanted into a smaller body like Jetfire, but in Starscreams case out of spite. The Decepticons really take no place in the larger story, saved for 6.

As Maximos massive world ship comes to earth, the group of heroes split into teams searching for Superion and the weapon. Vivians boss at the Royal Museum reveals himself as Thunderwing, an ancient pretender who served the Fallen and has now realized he needs to align with Maximo. He has been present throughout history, changing his appearance to fit the time. Through a flashback we find he was a Legionaire that fought in the Coliseum, a member of the Golden Horde, a Crusader, and on the front lines in both World Wars, which was how he came to Britain...he was essentially the basis for every mythical warrior in history. It turns out the weapon has been under their nose the entire time, a disc with odd hieroglyphs thought to be pre-Babylonian. Despite his size, he is more than a match for Hot Rod who is with Vivian, but Hot Rod is able to hit him with his time cannon and escape with Vivian and the disc in tow. Sam and Bumblebee get separated and found by Maximo himself and Maximo tells Sam that the small human is a bit legendary amongst the stars. He sent Lockdown to take bring him Optimus to help him kill Unicron, not realizing the planet Optimus guarded was what he was searching for. Maximo isn't evil, he doesn't hate humans, and wishes he could spare the planet. We learn that he didn't actually destroy the planet of blue skinned humanoids, by killing the elder as he fled, he stopped a plan that had been put in motion to self immolate the planet, actually saving millions. When he learned Unicron wasn't there and found his way to the actual location, he had no desire to bring any more harm. He lets Sam and Bumblebee go, and tells them to fight him with everything they have, die like heroes, and always remember it is nothing personal.

The battle at Stonehenge goes down like in TLK, but with hordes of Transformer size combat drones and air ships from Maximos world ship, with the world ship acting as the focal point to drain the energy from Unicron. Everyone searching for Superion is recalled to fight the present threat, they were out of time. But as the process starts, the ground shakes and the pillars of Stonehenge topple over, this was the resting spot of the Superion fighters. As soon as they reveal themselves they are swamped with Maximos drones and aren't able to combine but the distraction allows the group to push back to a degree. Eventually they are able to combine and they have air support. Vivian and Hot Rod join the battle, with the still unnamed alien giving her some weapons from his planet. Lockdown(now with blue optics, formally joining the Autobots), Hot Rod, and Bumblebee devise a plan and hitch a ride with Superion to the main ship. Lockdown is really the only one who actually stands a shot of going toe to toe with Maximo. They fight on the top, 3 v 1, but even then they are no match. However, with both Lockdown and Bumblebee under foot and ready to be killed, Hot Rod leaps at Maximo and the talisman activates and transforms in his hand, revealing itself as the Star Saber, impailing Maximo through the spark(a way to do an opening the matrix type moment without the Matrix). As Maximo falls to the ground and lays dying, he looks at Sam and says in the end, he is glad they survived.

Movie ends with Prime still missing, feral Maximo drones needing hunted, humans now having a world ship for defense, Thunderwing making his way to the Decepticon city, getting passed the defenses by taking the form of a TRF higher up and presenting himself to a living Megatron(who we see for the first time), and, most importantly of all but unknown to the heroes, a now awakened Unicron.

Part 6 coming later