Including the cars in the current TDU, I would like to see:
On-Road (rally cars also listed here)
-Suzuki Hayabusa
-Suzuki GSX-R1000
-Honda Superbikes
-Yamaha Superbikes
-Zenvo ST1
-2010 Challenger R/T and SRT-8
-2010 Camaro V6 and SS
-2010 Mustang V6 and GT (with Roush, Steeda, Saleen, Shelby, and Bullitt variants as upgrades)
-ACR SRT-10 Viper
-2009 Corvette ZR1
-1999 Mustang Cobra R or 2004 Mustang SVT Cobra w/ IRS (or both :D)
-Factory Five Racing GTM (LS7 version)
-More Gallardo and Murcielago variants (LP, SV, Superleggera, etc)
-BMW M3, M5, M6, Z4 M
-Mitsubishi Lancer Evo (8, 9, 10. Street and rally tunes)
-Subaru Impreza WRX STi (not current gen, either of previous 2 gens. Street and rally tunes)
-Focus RS (street and rally tunes)
-F-150 SVT Lightning
-Mazdaspeed 3
-Mazdaspeed RX8
-Mazda RX-7 Turbo
-Toyota Supra Turbo (MKIII and MKIV)
-Nissan GTR
-Neon SRT-4
-New Dodge Charger R/T and SRT-8
-Boss 302 and 429 Mustangs
-Chevy Monte Carlo SS (80's body style)
-Chevy Chevelle SS
-Nissan 240sx (S14)
-Honda NSX (either gen)
-Honda S2000
-De Tomaso Pantera (With factory racing body kit)
-2002 WS6 Trans Am
-Hyundai Genesis Coupe
-2009 Cadillac CTS-V (replaces old CTS-V)
-Honda Civic Si (current gen, U.S. and Euro versions)
-Ram SRT-10
-Mitsubishi 3000GT VR4 (Either gen)
Off-Road
-F-Series Pickups
-Ram Pickups
-Silverado Pickups
-Baja 1000 Race Trucks
-Suzuki, Honda, Yamaha, and Kawasaki Dirt Bikes & Quads
-CORR race trucks
-Sand Rails/ Dune Buggies (if we're going to be on Hawaii again, I want to tear up the dunes! :D)
That's all I can think of for now.