I think there is a way to make an Optimus / Cybertron movie work placed as a sequel to the Bumblebee movie and a Prequel to the Bayverse movies (since Bumblebee ended up being a prequel and not a reboot). Tying in anything to the Bayverse movies is going to be hit and miss because their plots are a shambles but I think you can make something workable.
My amateur thoughts on the way it should probably be approached is in the theme of Godfather 2. Showing two separate timelines concurrently where characters face similar situations in the past and present, the present being Earth, and the past on Cybertron.
The only way you can show character progression with someone like Optimus is to show how he became Optimus. So I think the Cybertron scenes should be centered around the progression of Orion Pax turning into Optimus Prime and the fallout between him and Megatron and how the war began.
Personally I really liked the way Transformers Prime portrayed the dynamic and backstory between Megatron and Optimus. How Cybertron had become a society riddled with inequality and ruled by an elite hierarchy, and how it was Megatron from the Gladiator pits who led the fight and championed a political revolution to bring in a a new order of equality, which drew in Orion Pax to his cause and together they worked together to bring about a better society, and it was only at the end after toppling the hierarchy when Megatron revealed his true intentions were to rule Cybertron himself that Optimus and Megatron became enemies and the actual war began.
I think this would be a great basis for the Cybertron storyline, focusing on the relationship between Optimus and Megatron and their struggle to bring about a better society until it reached the point where they became opponents on either side. The focus being on Optimus' decisions as the events unfolded and how those decisions or lack of foresight resulted in a broken relationship and a war which ended up destroying their society instead of bettering it.
Concurrently the Earth storyline should be focused on a human plot. If it acts as a direct sequel to the Bumblebee movie then it will take place in the 80's and can be centered around the ending of the cold war and focus on elements of espionage which means the transformers would be acting covertly. Basically there is a threat from the soviets which Optimus believes a Decepticon is behind. To investigate it he joins up with humans to try and uncover this threat. Because they have to work covertly it means that the majority of Earth scenes will be in vehicle mode which hopefully would balance out the CGI costs of the Cybertron scenes. I mean they are supposed to be Robots in disguise after all, all they would have to do for the Earth scenes is "Nightrider" the human transformer interaction (I really don't understand why the live action films haven't exploited this element to date, it would be the most cost effective way to portray human and transformer interaction) and who doesn't want to be able to talk to their sports car or truck like Michael Knight did with Kit lol.
Since the transformers are mainly restricted to vehicle mode in the Earth Storyline, the focus would be on the human element with Optimus prime and other autobots, although involved and assisting, being more of an Observer to the way the human events unfold whilst having flashbacks of similar situations faced in Cybertron.
By the third act both timelines reach a precipice, where the characters in both timelines reach a similar/identical decision which will determine the fate of each world. In Cybertron the decision made by Optimus caused the rift with Megatron and started the war that doomed their planet. In the Earth timeline Optimus intervenes and stops the human characters of taking the same path, which requires him to betray his human ally to achieve it for the greater good. This turns the humans against the Autobots and they leave planet Earth at the end as they were no longer welcome (perhaps showing the humans instead siding with the Decepticons), with only Bumblebee remaining due to info they discovered when uncovering the decepticon/human element about Megatron and the cube. This should then tie it in to the events of 2007 movie (with tie in's to Dark of the Moon as well).
Anyway those are just my premature thoughts for an outline of how both movies can be done as one, the mix between two concurrent timelines should see a reduction in CGI costs since earth scenes will be "nightridered" and focus more on the human good guy and bad guy interaction. We also see the background story of Cybertron and how the war started, and a character progression of a mostly flawless character.
Problem is it would likely be a long movie, and due to the complex storylines not necessarily catered primarily for kids.
Of course because the human element involves a spy/espionage focus, I would hope Jazz would play a pivotal role in the Earth storyline to redeem the terrible portrayal he got in the 2007 movie
My wife asked me why I carry a gun?, I said "Decepticons"... She laughed, I laughed, the toaster laughed, I shot the toaster. It was a good day.