Fully customisable offline championship mode
Its the thing that's been missing from every racing game for the last few years. People have been shouting for it in Project Cars and its still not been confirmed, even with PC3 in the making. A mode where you can choose your races and events, set the points awarded each race, choose circuits, change the names of AI drivers and teams (with a decent number, like 12 teams, 24 cars), etc. A sandbox basically.
If a racing game had that it would be an instant purchased from me.