My dream Forza would contain:
1. More tracks (could be even as DLC, I don't care). My wishlist:
A-1 Ring, Brands Hatch, Donington Park, Imola, Hungaroring, Singapore Marina Bay, Monaco, Monza, Shanghai, Spa-Francorchamps, Abu Dhabi, Zandvoort, Lausitz, Donington, Oulton, Daytona, Slovakia Ring and few more which I can't remember right now.
2. Qualifications as an option to choose in race settings.
3. Ability to race with SI for more than 2 laps without having to purchase Xbox Live Gold.
4. Dynamic skies/weather changes during race (i.e. 30 lap race starts with clear skies and sun, and then it gets cloudy and starts to rain). This could be an option to choose in race settings - "Allow dynamic weather changes Yes/No".
5. More vinyl/painting options (vinyls on all surfaces with no restrictions, brake calipers painting).
6. Realistic physics off-track (I know the "glue sand" is something that holds off cheaters from cutting corners, but something else could be applied, i.e. cutting corner through the grass would result in a penalty drive through the pit stop forced by the game, or if it would be the last lap - a time penalty or a ban for online races for 1 day or so; this would have to exclude situations when someone pushed you away from the track or you went off it as a result of an accident).
7. Better damage model on Simulation settings (so hitting a wall doing 100mph would result in total demolition of the car and the end of the race, more parts falling off the car when in accident - doors, wheels, hoods etc., maybe some oil/coolant fluid spills).
8. Some cinematic career story with your Xbox avatar climbing up the 1st place box, getting a champagne from a nice girl. :-)))
9. Parts, parts, parts - also visual tuning ones, like in NFS, some body kits, more spoilers, diffusers, more alloys, the sky is the limit!
10. Better in-cockpit animation, including the outside mirrors - can we have them adjustable please, like in some trucking simulators on PC?
11. Live pit-stop with moving mechanics (this is something I remember from Amiga 500's game about Formula 1 - I cannot recall the title - there was a guy waiting for you with the lever to lift your car... Aaach, old times...). Repair times adequate to the damage of your car.
12. A guy dancing with the black-white checkered flag at the finish line. :-)))