I'd love to see a few more options of Subarus in the game... there's about 5-7 Evos and only 3 Imprezas... I'd like to see some of the J-spec STi Wagons, maybe the bug-eyed 02-03 (IIRC the 00-03 models in Japan) STis.
Also, the Dodge Challenger, both old and new... meaning the 1971 Challenger and the 2008 Challenger. Some rediculous sedans would be cool, too... like the Taurus SHO (GT4 HAD IT!) and the Dodge Intrepid R/T or Eagle Vision TSi or 300M (GT2 had it!) and the Pontiac Bonneville V8... that thing COOKS.
As much as I dislike the 80's fox-body Fords, the inclusion of them would be pretty cool: like the 5.0 Mustang or the I4 Turbo Mustangs and I4 Turbo T-birds...
the inclusion of certain SUVs would be neat, too. Not like... Jeeps and Hummers... but stuff like the Volvo XC70, Subaru Forester STi (I think it's still only J-spec and GT2 had it!), Porsche Cyane (or however it's spelt).
And on the odd-ball lists: Australian 60's and 70's muscle cars (MAD MAX!) like the Ford Falcon XB GT, XB and XA sedans, the Holden HQ Manaro... man... those Aussies had some awesome cars.... and more Korean cars, like the Hyundai Accent Evo or Elantra GT, maybe Kias or Daewoos, just for S's & G's... it'd be fun!
edit: and I'm with Kelz, I'd like the Sentras from before they Saturn Ionized them.