I'm quite interested in this as well, I have done a Transformers Rp before and am a massive fan of the Transformers Universe, both in G1 and other continuities.
There was of course the War for Cybertron between the Decepticons and the Autobots, the war seeing a young Orion Pax becoming Optimus as his body was rebuilt into that of a soldier from that of the Iacon Archivist he started out as and then seeing him become the Prime that we all love and respect. Then there was the crash landing of the Ark in another continuity and then the volcano exploding and reactivating the troops within of both Autobot and Decepticon origin (Generation 1 Transformers) There is also the previous friendship between Megatron and Optimus while Optimus was still Orion Pax. Megatron tried to manipulate the young Autobot into decripting the Iacon database so that Megatron could find the locations of several weapons that would have given him the military might needed to crush the existing Autobot order and rule Cybertron. As it was, these weapons had been removed from the planet and war waged until the planet was eventually rendered uninhabital, forcing the Autobots and Decepticons to vacate the ruined husk of their homeworld, searching for new planets to call home or dominate.
Optimus Primal was not Optimus Prime but a deep space explorer with a similar name, he took on many of the qualities of the original G1 Prime, but was never actually a descendant of the great Autobot leader. In one of the later seasons of Beast Wars, Beast Wars Megatron infiltrates the Ark and tries to eliminate Prime before the Great War starts, thus allowing the Decepticons to win and the Maximals to have never existed in the first place. However, Primal and the Maximals are able to save Prime and thus the Generation 1 timeline is preserved.
There are several other continuities as well, but the easiest one to follow is Generation 1. I can help you out with any character traits or problems you have as I love Transformers and have quite a good knowledge of the various characters on both teams. I would also love to play and original character if you would be interested in having me aboard?
As for RP ideas, I would love to do an RP set within the War for Cybertron, with the young Optimus and a Megatron consolidating his power and his forces. Megatron gathers his forces in the early days of the war by taking control of the Dark Energon research facility, bringing Starscream into his ranks, while also giving Megatron a massive power boost when he absorbs the Dark energon, becoming the first being capable of weilding it's unimaginable energy levels without being destroyed from within.
Optimus is the young and experienced soldier, but is surprised and unsure when he is appointed the position of Prime when Sentinal Prime goes missing in battle. Optimus leads a small force into the Decepticon city of Kaon, where they find and free several Autobot captives, eventually finding Sentinal, but it is too late and he dies, making Optimus the last of the Primes.