The career mode got a lack of interest because there aren’t any objectif.
If we begin as driver in low categories who race for teams in way to have money to create our teams and climb into big championship it would be more interesting(The kind of things donne in race driver grid).
We could have a reputation system wich allow us to work with great constructor if the reputation is good enouth. You could sign exclusivity contract with them for some category or some races/championship which will bring us more rewards and exclusive parts.
The different championship should take place in a calendar so we have to chose each years few of them where we can compete.
Maybe a system of promotion/relegation where hight categorie championship are locked if we are not in good enough division should be a important part of the career mode.
It 's the kind of things career mode need to gain interest.