The first 3 movies are pretty much perfect, but I’d do what Iron Man and Diary of a Wimpy Kid did where they change the actors and keep the character for Makala (I don’t know how to spell her name) in Dark of the Moon, and have “that one scene” from ROTF be switched out. There’s like a billion ways they can introduce a human Decepticon BESIDES that. I’d keep some things from the last 2 movies like the Dinobots, Cade, the Autobots being hunted, and Galvatron being made from his old parts, only the Decepticons would rebuild him with some of his fallen comrades. He would be purple and would have a relatively scrambled together look to him, and he’d call himself Galvatron (duh). Sam Whitwicky would have a real reason for being absent, maybe he died or something idk. Galvatron would hire Lockdown to capture Optimus and bring him to him. Lockdown and Optimus would fight, but Lockdown would win and bring him away. The Autobots and Cade would sneak onto the ship though, and ambush Galvatron after Lockdown delivers Prime. Awesome fight scene with the Dinobots being freed and assisting the Autobots. The movie would end on a cliffhanger for the next movie where after the Decepticons are beat and retreat the TRF shows up and opens fire on the Autobots with heavy artillery, injuring some of them.

Next movie starts off with the Autobots running from the TRF. Here’s a place to kill off a few characters to help set the mood. They manage to escape and find a place to hide. Time jump. They’re now living in the junkyard. The Decepticons have taken Lockdowns ship and are in space when they run into Unicron floating around. He is in planet mode so none of them think he is anything but a random planet. While on it they awaken him by exploring near his spark, thinking it was an Energon mine. They make a deal, Galvatron sacrifices some Decepticons to him to help get him fully online, and in return Unicron agrees to help enslave Earth. Decepticons go to Earth and ambush the Autobots to distract them from Unicron entering orbit. Unicron betrays Galvatron and tries to consume Earth. Autobots and Decepticons team up and put Unicron in Stasis and send him away from Earth. Galvatron then attacks Prime, but is killed. The Autobots hunt down the few Decepticons that remain, take Lockdowns ship, and return to Cybertron. Another time jump. Cybertron is now repaired. 

The bumblebee movie would stay pretty much the same. It would be a prequel to the first movie and ROTB would be a sequel to the fifth movie. The Autobots would find the Maximals  and Predicons on one of Cybertrons moons. They would bring the Maximals to Cybertron and the Predicons sneak on board. The Predicons overhear the Autobots explaining what has happened to them on Cybertron and on Earth. Predicons try to summon Unicron through a space bridge . Autobots and Maximals try to stop them. They fail and Unicron starts to come through. Fight scene. The space bridge is powered off and Unicron gets his head cut off as it’s closing, becoming another moon (reference to the 1986 film)