For me, the most obvious is Porsche. EA are assholes for not alloing other developers use Porsche in their games, and they are my favorite car manufacterer. I would specifically like to see the Porsche GT2 RS, Turbo S, GTS, Carerra GT, GT3 RS, and the base 911. I also wouldn't mind seeing the exact same cars for older generations (996 and older) to see how much well a modern base 911 compares to a 10 year old Turbo or GT2.
I don't like how the BMW 1 series M Coupe was a Best Buy exclusive pre-order, and I would like all exclusive pre-order cars to be available. Base Cadillac CTS, Lotus Evora S, Benz C63 AMG Black Edition, the late 90's Mercury Couger (not the old one), and well the only super car I can think of, the Ariel Atom 3 and 500 V8.
Also I would like to see more vairents of different cars. An example would be the Lotus Elise, there are only 2 of them in Forza 4, and no supercharged version. In the 15 years or so it's been on the market, the car has seen 1 major facelift and numerous engine changes. I would like to see a few varients. Namely the 50MPG 140hp model, the Elise SC, and some of the older ones with 140hp or so. Some 80's Toyota Supra's would be nice too.
Would also like to see some more concepts. The DOME NSX is a big one, so are the Toyota FT-86, the BMW Vision, Lotus Concepts (Elite, Elan, Elise, Esprit, and Evoque). Forza isn't known for concepts, and in the past they almost had none, but I wish there were more of these.
Some old cars, the 2003 Acura CL Type S, that 1931 Bently 8-Litre (would be fun to race with), Jay Leno's 1 off car (forget the name), the first BMW M3 (they have all the other ones), and well any 1980's M car, or 1970's-1990's Benz with a large displacement (SL's, SEC's, CL's, etc), the base Corvette C3 (not the ZR-1), and the first Lotus, the Lotus Elite.
Overall, more concepts, and some more variations of different models. I personally think Forza 4's car roster is excelent, and basically all the main cars are included. I'm just nit picking. Seeing the Bentley 8 Litre in Auto Vista, and some 100 year old benz's in GT5 made me want to see some older cars in the game. I mean the car economy was so different 50 years ago, Lamborghini didn't exist, there was no Porsche 911, Japanese cars were almost non existant... Would be cool.