Well.....that would make sense for the add-on....add-ons. Given the way games, or at least all the good ones, are built today, I would expect that SR3 to have a lot of add-on cars, parts, and tracks to expand it. However, as I understand it, we're talking about the standard, unmodded game here.
The plugs will come later, unless we want to build it piece by piece, which isn't entirely a bad idea necessarily. Trying to do everything at once could potentially delay its release indefinitely because we think that one crucial thing or another is missing.
For example, if I get voted down on the Vega, it could be a plug that people could choose whether or not to install, ditto a VW Beetle, Porsche 356, or Corvair, all of which break the mold by being rear-engine and non-V8 in stock form, with the Beetle and the Porsche obviously being German as well. If we go with a plug-by-plug development model, it will enable faster decision making in this way as we won't be arguing over every little detail.
I suppose the "standard" plugs voted on by the majority would go into the "built-in" folder for the game. I'm only speculating, but I would assume these would be cars revived from SR2 or SR1 like the Ford Model 18, the Plymouth Belvedere, and the Chevy Impala (whatever year), as well as bog standard muscle cars like the Mustang Boss, Charger, Camaro, GTO, and 4-4-2. Meanwhile, the oddballs like the A/C Cobra, Vega, or Corvair could be plugs.