Ken Christiansen, who did a series of art pieces for Hasbro when Hasbro were concepting the look of Transformers Prime and the Aligned Continuity, has posted up concepts of an Optimus Prime design he worked on. As he explains in his post on The Bad Flip Blog
, sometimes he was only commissioned to design one mode for the characters, in this case, he was only asked to do concepts for the Optimus Prime truck mode. As you can see, there are a lot of similarities with the design that was ultimately used for Transformers Prime - although this version looks a little beefier with a slightly deeper cab and elongated roof as a result.
Check out the concept at the link above. Ken Christiansen will be attending Botcon 2013 as a guest in their Artist's Alley - if you are attending Botcon be sure to say hello to him and thank him for sharing all this cool concept work with us.