After having picked up a clearanced TLK Megs, it got me started thinking again.
Megatron has been the one consistent element of the movie series. Every movie he gets a new design, and that's actually really rare. And further still, not only does he get new designs, but they are all very recognizably him. And the story reasons behind his new forms are the most grounded and logical parts of the movies. The fact they are all very recognizably Megatron was an entire plot point of AoE, even!
So... we get to The Last Knight... and the new Megatron has absolutely no connection to anything we've seen before. There's no design logic, or story logic to suggest why he changed from OptimiClone Galvatron, to giant armored demon. There's no visual cues that link him to being recognizable as Megatron. They could have easily applied any random name to him. Not only that, he doesn't really play a role in the film. He's more or less just there. He comes out of the blue, gathers some troops, and leads a charge. Honestly, Barricade plays a much more important role.
I don't think he was designed or meant to be Megatron at all.
It would be in keeping for Megatron to be unhappy a; looking like Prime and b; being in a body built by humans.
So the change itself didn't trouble me. As to the form itself I see hints of previous movie Megatrons. A jet mode from 2007 and the big arm cannon from ROTF. The look, maybe he was just as inspired by the Knightly look of the guys on the bounty hunters ship as Prime and Bee.
You're right though, Megatron did have more continuity/developement in the movies than really anyone else. That never occured to me.