Not going to repeat anyone else's ideas on career modes and the rest because there is alot of good idea's.
And as its a "Driving Sim" money no object, I would go all hardcore, And yes a total pipe dream that will never be met anytime soon
Team up with Google maps/earth, And have a 1:1 scale earth as close as possible to real life.
If its 350 miles between two tracks you have the option to drive that real road, load up your truck and off you go
ferries, cargo ships, planes included, employ other players, And a bonus to that is it appeals to flight, shipping and truck driver sim players ( Big fan of Euro Truck Simulator myself )
If you want to drive around the globe in a Daihatsu midget, See you in 8-10 months haha.
Add properties, parts warehouse's and factories, dealerships, for the tycoon types
Suspension rigs and full on engine design workshops
(Think automation, A WIP PC game, where you design an engine from scratch bore, stroke, internals like picking if you wanted cast or forged pistons, cast iron or magnesium block, fuel ratios, compression/cam adjustments) then sell the engines as crate motors to other players, To fit into player designed championship regulations, Test track/Trackday costs, for the tuners
Empty land for sale for track builders that would need to be managed and designed well
Say I build a 4 corner oval very similar to Indy, no one uses my track, Because well Indy is better I go bankrupt and lose the land for someone else to buy and build on, And another player builds an epic original track that alot of players enjoy they make money keep their land, And get to upgrade facilities to make it even better, Unless they go AWOL and don't manage it for a while they lose the land to.
Full on weather and time of day
So if its night time and raining at Suzuka in real life its night and raining in the game,
or any other real world track.
Player designed races where you can stop at a hotel/camp site save your game and come back to the race later on
Trophy truck race from Melborune - darwin, 43hours and 3,752 km, Right across the outback,
GT road car race from Lisbon, Portugal—Shanghai, China ( I broke google maps trying to work out how long that would take haha )
Rock crawler race to the summit of Mt Everest.
Huge groups of players hanging about at places like, Tail of the dragon North Carolina–Tennessee, Shuto expressway - Japan.
Buy/rent houses and hotel rooms for fast travel between favourite locations
I could go on forever but i'll stop here cause I have blown my budget of a few billion already ;P