So if you've watched TFA you know the Prime is a military title, but what if it wasn't just a military title but the title was stolen and used as a military title to get rid of the history of the 13 Primes of Old (the 13 original)
So Primus wakes up or something and sees how shitty Cybertron's become and decides to reincarnat his first creations (with there consent of course) and he knew Alpha Trion would be extadic to have his siblings back. And of course Optimus is the reincarnation of 13, but has to hide it till the time is right. Cause due to the fact of Cybertron barring the 13 if ge revealed himself to the council he'd most likely be killed or something.
So it just goes like this
Optimus; on earth thinking: I know creator, and Alpha said to keep it a secret but it wouldn't hurt to tell them right? I mean we're not on Cybertron.
A few minutes later
Optimus: Ratchet, I really need to talk to you privately.
Ratchet: Sure kid whatcha need
Optimus: Well..... do u know about the Primes of old?
Ratchet: I've heard bits and peices why?
Optimus: ok so thus might sound crazy but I do have proof, I am the 13th prime. *Optimus reverts back to his original frame.
Ratchet: Holy scrap kid.. if anyone on Cybertron finds out you could be- be
Optimus: Killed, or imprisoned I know, but I feel like I can trust you Ratchet.... don't make me regret this please.
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Like just imagin if Megatron knew about the Primes of old and only respected then and found out the one who could defeat him has the youngest true Prime, and that Optimus/13 was possibly holding back so he didn't get discovered. It would be better if one of the Decepticons were a reincarnation of one of the 13 original Primes.