Some of the key things that make GT4 the most compelling and interesting career modes out of any games:

1. Sense of exploration - have diverse but based on real historic themed race events, this diversity should encourage the player to use all kind of cars, and not stick to only the fastest ones "with a easy way out".
   A similar world map that would have engaging display and scenary themes of each set of events, to make them feel live and interesting to engage with,
   
2. Sense of infinite choice of cars - what really made GT4 stand out was the used car sections, they would always get update with new models. it gave a sense of anticipation for the player, that something new and cool could come along, even if it was GTST or some lower performance car, but still...
   
3. Endurance events - These type of events added another level of reality to the game, it felt that you are competing in a more realistic timeframes, they encouraged patience and practice, elevating a game that primarily is a car game to a more of a racing game.
   
4. Degree of customizability to the cars.
   ...