I can picture this happening, but I think it's far more likely that they had Optimus in mind from the get-go. Especially since he was going to be front and center for Aligned and particularly Prime, the show that would supposedly explore "what it means to be a Prime".
The thing is, the Exodus novel (which was the one piece of Aligned media that was the closest in spirit to the original backstory described in the Binder of Revelation) leaned heavily into the idea of Orion Pax being just an ordinary guy who was called to be something more, not because of any preordained divinity, but because he was just that pure-hearted and likeable. At no point anywhere in that novel, in its video game adaptation WFC, or in its successor novel Exiles, is the idea of Orion Pax being the reincarnation of an ancient holy demigod ever hinted at.
The Prime cartoon even had Optimus refer to the Thirteen as though he was a completely different Prime from all of them. And the show really ignored a lot of things that were set up in the lore of the Binder (Unicron being the biggest deviation, as he was
not supposed to be Earth at all). The cartoon really wanted to do its own thing. The main reason anything from the Prime cartoon matched up lore-wise with any of the novels or video games was because of Mairghread Scott being part of the writing team, as she was the only one on the team who actually read the Exodus novel and had any real familiarity with Transformers lore in general. The rest of the team was largely made up of people who were not as familiar with Transformers and that was done on purpose, because the showrunners wanted Prime to have more of a fresh perspective that would try new things instead of retreading old ground.
As for why I feel like The Thirteenth was originally going to be the preincarnation of Rodimus while Optimus was going to be akin to Prima, well... Let's just say I've read some things that cannot be publicly disclosed at this time (PMs on the other hand...)
Anyway, backing up a bit to earlier in this thread, regarding Rodimus's character arc in season 3 and how it could have been better structured, let's face it: we were never going to get anything better structured because character arcs really weren't a thing in 1980s American children's cartoons. Serialized storytelling was rare outside of multi-parters, and season-long arcs were really only a thing on American television in soap operas and anime dubs (and that's only if the dubbers were able to recognize those arcs and faithfully adapt them into the English version). The earliest American-made children's TV cartoon that I can think of that actually had season-long character development arcs was
The Legend of Prince Valiant, which didn't first air until 1991.
Sure, we got cases like "The Search for Alpha Trion", "The Key to Vector Sigma", and "War Dawn" containing interconnected plot points across the second season of the G1 cartoon, which were then built upon and further expanded in the season 3-4 episodes "Five Faces of Darkness", "Forever Is a Long Time Coming", and "The Rebirth", which all certainly gives the
appearance of an ongoing narrative. But none of that was planned out from the beginning, and a number of connections between these episodes were made after the fact by the story editors instead of the individual episodes' writers (particularly Ariel in "War Dawn" was never originally planned to be pre-Elita One; she was supposed to die along with Dion). Everyone was basically making things up as they went along, flying by the seat of their pants and throwing things at the wall to see what would stick. And that included Rodimus's portrayal across the third season.
