The ultimate game would be one of three parts, sort of like how there are different Pokémon games.
Formula-Spec would have you start in karting, advance through the ranks, offered drives, going through Formula Renault/GP3, World Series/GP2 and into F1 with a career mode like the Codemasters games.
America-Spec would have you start in karting, advancing either to US F2000, Indy Lights and IndyCar or through stock cars to NASCAR Camping World Truck, Nationwide and Sprint Cup series
Touring-Spec would again start in karting, before you rise up through the GT Championship or the ALMS, finishing in the soon-to-be-created FIA Endurance World Championship with Le Mans 24hrs at the top (could possibly fit in the BTCC/WTCC in there too).
All 3 games would be separate, but completing the game will unlock the ability to download content (probably for a small fee) to use your game data to “switch” to the top tiers of the other games’ car classes (so you become F1 World Champion and then switch to IndyCar like Mansell, or try your hand at Le Mans).
Totally impractical, but a man can dream.