I think this is an excellent idea for a game. There are a lot of opportunities that you have created for yourself to have a lot of additional elements beyond what is experienced in GTA, so you have a true foundation from which to create a more dynamic game with more gameplay depth than GTA. Truckers, biker gangs, and police is a great combination... like traders, pirates, and bounty hunters. It is a classic dichotomy that has made for many classic games. I am seeing this as about half-way to a road warrior atmosphere, being an alternate universe 1970's with all of this highway combat.
As additional inspiration you might look at Steve Jackson's Car Wars, a classic of the old board game industry. Also the better gangster games, the more strategic ones and not the FPS ones. Aspects of those games might inspire you in this.
I really like it a lot just from a story and content perspective alone, but it really does also add a wide array of tangible things that can make give it much more gameplay depth than GTA.