The following is basically an idea for the start of the campaign in the game which I personally think would be quite good, anyway here we go:
You start the campaign in 1940, now as most of us know this was the start of World War 2, which most nations played a part in, therefore I think this would be a good theme to start off with. At the very start of the campaign after you’ve chosen what nation you will be based in and name etc…, there will be a brief storyline of how possibly you and a brother/friend/father (or vice versa if you are a female character), was extremely interested in car design and engine design and you have slowly gathered together a small workshop with your partner, enabling you to be able to build basic engines and cars to fuel your hobby, now because of the rise of the war effort in your chosen country, word got round to a local administration office of your hobby, and gave you a contract to build a basic engine that could be used in a tank (this part can obviously be changed on what kind of vehicle to put the engine in), and then this would lead into the engine tutorial. In the engine tutorial obviously it would give the player a tutorial on how the engine builder works and so forth, but it should also guide the player into building the exact correct engine for the contract they have been given, so once your prototype engine has been built, it is shipped to the administration office or some sort of army department, where it is critically analysed, and funnily enough perfect in every way for there requirements, so they then give you a bigger contract on producing these engines, and yet again another tutorial would be put in place for this section, and then because of the success of your engines this is how your hobby has now become more into a business by making your first large sums of money from the success of it.
Now later on in the year (1940 still), a competition between all car/engine manufacturers of that particular nation is staged, to build a new luxury car for the leader/monarch of the country in aid of the war effort (like in terms of moral and national pride I guess), and obviously as a small coach-builder you accept the challenge, and then this leads into the car design tutorial, and then a small tutorial on building appropriately sized engines for different car bodys etc…, and then tutorial on gearboxes, wheels, brakes, suspension etc…, and then finally an overall tutorial on production. Your entry is then entered and based on statistics generated from the game, as this last part was more open to the user for designing, you will either win it or come second or third (basically so they still get a bit of a financial gain), and because of the success of your initial engine and then the success or relative success of your first car, you are now free to build cars and engines, or accept contracts/grants at your discretion.
So that is my basic idea, its a bit holey here and there, which is ok I guess as it’s just an idea. Anyway I would look forward to hearing feedback as everyone has different opinions. And finally thank you for taking the time to read all of that. 
more on that once it has been fleshed out.