I'm itemizing my thoughts:
No NASCAR game I've ever played has had spot on physics, BUT NR2003, DtD, and Thunder 2003 & 2004 have feels that aren't too far apart from each other, so not having in-depth knowledge of the cars you can exchange them out and probably be close.
The AI is important because while it needs to be competent at worst and legit at best, it also needs to be human and make mistakes, just not the entire field and the exact same thing for every driver (EA).
Structure wise, again take your pick of old games. I can't make a preference here because the four main benchmarks did their own things well, so it comes down to preference.
Substance wise is different. Any driver/any track/anytime for single player and/or multiplayer is a must, as is a pick-up-and-go season mode, but one thing I've figured out playing with NR2003 is once you take away the PC advantages it kinda falls flat. I think a fusion of DtD and Thunder career would be ideal, but no '20 season limit' that the Thunder games had.
Being able to readily mod it would also be great.