My suggestions to NFS modders is:
From NFS:UG or NFS:U2, the Mazda MX-5 (NB), with upgrades, damage and everything, so it could compete with the SLK and Z3 in the class B.
From NFS:U2, the Peugeot 106 GTI. Perhaps with upgrades bringing it closer to some sort of rally version so it could be more competitive. Oh, and if it were turned into a 106 Rallye, that'd be even better.
From Forza Motorsport, whichever of the Toyobaru twins/triplets you guys want. It's been a while since the car was released, and since it was featured in games, so I'm actually surprised nobody has done it so far.
Take note that these are suggestions, not requests. Also, I'd do some of them myself if I had more time and will to do mods for the game, but then, I don't remember the last time I played High Stakes.
EDIT: One more I forgot about. From Real Racing 3, Porsche Cayman GT4. It'd be perfect if it had upgrades, better yet if it somehow had a dashboard.