As I've said elsewhere, I'm not a coder so much, but am an enthusiastic SR fan of 23 years. Having played all three games this week, these details come to mind...
-Following the modern-day trend in gaming, SR3 should probably have the option of never ending, so that people can build their cars indefinitely, particularly for the proposed multiplayer mode. SR1 and 2 were very 1980's in that you could finish them in a few hours, but this is disappointing today.
-Speaking of that, multiplayer should be done right (a reliable way to host games without a central server, Teamspeex-style communications, etc.), both to make the game more enjoyable and to attract additional developers. In open source game development, social networking is crucial, and at the moment this project is suffering from a lack of participants.
-The decals should include flames. This is obvious, isn't it?

-Fix the sound, and please don't add cheesy music like in the SR1 and 2.

Don't worry, I know you're working on this. Support for user MP3's via the radio is in line with what was done with Redline, and would be more welcome. Redline actually took this a step further by being integrated with iTunes.
-In SR2, Chevy lacked an entry-level car. It seems obvious, therefore, that SR3 should have a Nova, a Corvair, or both. The Corvair would require a flat-six engine and a turbocharger, but these would be easy to make. The weight distribution should be made to be accurate, too, but that I assume will be in the new physics model. The Nova, however, would be the more important of these two, as it was a favorite with hot rodders.
-I noticed that the alpha does not yet support 6-barrel carbs, triple intakes, superchargers, or fuel injection. SR2 at least had FI and superchargers, so I assume it's coming. I'm just making sure.... Nitrous would be another obvious mod to add.
-I've already "built" a Pontiac Tempest for SR3, and I can do a Plymouth Satellite, too.

Please, let's not have a 6-cylinder Deuce, GTO, or Roadrunner.

-All three SR versions fail to accurately depict the speeds involved. SR1 and 2 limit you to 120 m.p.h., which is accurate for a 1950's American sedan or wagon like an original Crown Vic or Country Squire (or something with a 4.11 differential), but far less so for a Corvette, Chrysler 300, or Charger. One variant of the real 300 could hit 160 bone stock, after all. The alpha of SR3 is just the opposite, in that a car with a "small" engine can blow away something with a "large" engine, reaching speeds in excess of 160. This will probably be fixed in the new physics model, but is worth remembering. Modern car simulations, even NFS, can easily get this right (even if the NFS cars are not accurate depictions of the specific real-life cars), so SR3 should as well.
-The same as the above should apply to things like burnouts, donuts, torque steer, racking, body roll (crucial for depicting American cars of this era), and spinouts. Amazingly, SR2 could simulate drifting, if crudely and without wheelspin, which is impressive when you consider SR was the same era. Neither Test Drive version of the 1980's (1 and 2) could do this, although TD2 could do slides. However, hot rods aren't much fun with NFS-style physics. Redline was far better than NFS, TD, or SR in that a six-cylinder was really bland, while an L88 could burn through all four gears, write or draw on the pavement in skidmarks, throttle steer sideways, and envelop itself in smoke. That, I think, is exactly what we want, plus loud and realistic sound.

Redline also had handbrake simulation, which would be a cool thing to have in SR3.
-Although traditional hot rodders scoffed at spending money on good brakes, they are a realistic part of racing. Hopefully there will be ways to upgrade them?
-Modding of the game should be made easy, but this is obvious nowadays, isn't it?
-I noticed that SR2 used to always have an ad for a Desoto in the margins. Perhaps just for the fact this car was so ridiculed.....it should be included.

I could make it, if I have the time. A Studebaker, Packard, Hudson, Nash, or Edsel would be interesting too, although it would be possible to get carried away...
-Every big-three brand was in SR2....except Cadillac, Lincoln, and Buick. The model choices here are obvious: Eldorado, Continental Mk.III, and Electra 225 (although a Riviera or '53 Skylark would also be great).
-Basic engine blocks to add (as of the SR3 alpha): Pontiac 400; Ford 221 (for the Deuce); Chevy 283, 327, 350, and 427 (for the cars already in the alpha, plus the classic 327); Mopar 318, 383, and 426; Ford 302, 427, 428 (for the GT500), and 429; Olds 455. If a Cadillac is eventually built, having a 500 would be great, too. I presume that add-on developers could add dozens more of these, but I think the above would be ideal for the built-in engines.
-More transmissions are needed. This is something obvious, but the built-in variety needn't be immense. Seven trannys would do if game modding is easy, as each of the big three needs an auto and a 4sp, and there needs to be the existing 3sp for the Deuce. I've already done an M22 4sp., a T10 4sp., and a Powerglide, and the first two of these could be any manufacturer, so theoretically the next alpha could get by with only three, the Ford 3sp, the T10 or M22, and the Powerglide.
-Crate engines, anyone? They already existed in the 1960's, and Chevy in particular built a range of them, including the 377, the original 409, the 427 HD, and the 433 CanAm. Indeed, hot rodding was the only thing that crate engines were for.
I think I'm out of the obvious things for now...