Origins wise, I have 3 criteria that I have mentioned before: 1). Commoner background. No nobility, no authority. He can already be an inspirational figure to the proto autobots, but he shouldn't represent the power structure of the prewar era. Let the war be the crucible that makes him into Optimus Prime. 2). War Dawn relationship with Megatron. No longer are they former friends. Instead, give them a moment in which Megatron's savagery becomes crystal clear to Optimus and let him develop from there. 3). Having given him a starting point and a turning point, proceed to allow him to rise through the ranks until he becomes the hero Optimus Prime. No chosen ones, only proven ones. For personality, the most important things to get right are a genuine love of the world and all there is to learn about it and that he is in Peter Cullen's words "strong enough to be gentle".