Optimus Prime was a Mack truck in the cartoons, but in the Transformers movies he switched over to Peterbilt. And if the Peterbilt brand is good enough for the leader of the Autobots, they're surely worthy of your design attention.
Peterbilt Motors, in conjunction with (Core77 Design Awards Transportation Judge) Local Motors, is holding a competition to redesign their next-generation Peterbilt Icon cab. Entrants are asked to incorporate traditional Peterbilt styling into the new RIG2 (Road Icon Generation 2), to "Provide a design to stir the emotions of the owner while still communicating practicality and functionality."
The competition's splash page is pretty helpful, spelling out the terminology of the parts entrants will need to be fluent in, and both Photoshop and CAD models are provided to serve as underlays.
Here's a quick look at how you've got some wiggle room with some of the underpinnings:
There's also a series of graphics demonstrating aerodynamics, as they're clearly looking for something swoopier than the boxy current model.
As thorough as it is, the brief mentions nothing about anti-robot battle-worthiness; so I think we can safely assume that, as seen in Michael Bay's latest documentary, our world has been rid of the Decepticon scourge forever.
Check out all of the RIG2 competition's requirements, guidelines, and deliverables here.