These are things I would like to see:
1) Engine Tuning: THe ability to choose engine parameters. Like a choice of Cam profiles, or a tool to create our own cam profile. A choice of valve sizes, or a tool to create our own. A choice of engine dispalcement, Rod length, crank stroke, piston size, .variable intake runner length/diameter, variable header primary/collector legnth and diameter. Turbo size (creating higher boost or lower lag), Turbo wastegate parameters. The goal of all this would be to allow the gamer the ability to create their own engine power curve(torque and HP vs RPM). That would allow a player to make a torque monster, or a revver, or a mid rpm screamer, for different tracks and purposes, and add a new dynamic to the game. THis sort of modelling has been going on in computers for almost thirty years, it would be fairly simple to add to Forza.
2) Car Ideas: Mosler MT900, 1970 Plymouth Baracuda AAR, 1970 Challenger T/A, 1970 Chevy Camaro Z28, CAN AM cars, Lotus S7, Lola T7, 1989 Mazda RX-7 GTU or Turbo II, Porsche 908/916, Jaguar Type D, MG's, American Saloon's from the 50's (55-57 Chevy, 58 Chryco Imperial, 58 Ford), more early Feraris (1958 Testa Rossa, 1962 250 GTO)
3) The ability to choose racing conditions: Rain Tires would be needed, obviously. working headlights, with additional lighting would be needed. Night racing, inclement weather (lightning would be a super cool effect to see during a race)
4) More tracks from outside USA/Europe/Japan. African/Asian rally courses, Mexican Road Race courses (that should be a whole extension unto itself, they could model all 2000 plus miles of the race course, and add in all of the game features they miss in the release of FM3)
5) Did I say more engine tuning options???
6) The letter/number classifications work really well until I come up against a mostly stock high A class car with a car that matches it in class letter and number, but mine is fully tunable. Unless I really choose wrong (unlikely), or make multiple driving miscues (much more likely), I will have the race in the bag. This disparity might need to be addressed, but I don't know how it would be done without mucking things up.
My two cents...