If I were designing it , and I was also forced to make sure that the following elements were used - all 4 party members , a sense of adventure, visible weapons to imply combat , floating shit in the sky to stir mystery in what lies ahead , hell perhaps even a visible road and most importantly - the freaking car - I mean, this should have been a no brainer.
You place Noctis in the center , holding his sword aloft / skyward. Gladio to right, Ignis above and Prompto to the left. They are all depicted smaller, there's no scale here. All of them holding their weapons.
Logo is on upper third of design , SQUARE logo on lower right, as is standard Background should have a bluish lower half depicting Insomnia , showing small heads of Regis and Cor , a split down the middle is vaguely a highway , Luna is on the right side (maybe her handmaidens head is next to hers and then on the upper left looking down is the emperor and his adviser , the upper portion of the background showcases Lucian drop ships and the far background an army of vague shapes meant to represent magitek soldiers. The far back ground could still maintain the knights of the round clouds.
I'm far too lazy to actually make this up though. It would look even better if they got it drawn up classic hollywood movie poster style.
As for the car ? well, just use the japanese box art as the back cover art but have all 4 bros sitting next to the car with a couple of screenshots and the tag line "an epic road trip adventure awaits".
In all honesty though, plain black background with logo centered would be my go to option.