Blur and Split/Second used the car as a sort of HUD (Split/Second moreso) by 'sticking' important things (in this case weapon icons and stuff) behind the cars. So having the weapon icons following the car directly could be possible?
Though, I'm hoping that we get some sort of sick jet-engine aftertrail, which might get in the way of those icons.
Maybe an option to stick the weapon HUD in the center-bottom of the screen as opposed to the far-left would be a good idea? Then it's always right beneath the car and you don't need to look anywhere. Choices, choices.