First, this game was worth the money. Me and my little brother love the game and have lots of fun making the cars wreck as much as possible. Now, on to the suggestions.
1. Asphalt ramps / bridges
The dirt jumps / hills are arguably one of the best track pieces in the game, but theres no asphalt or speedway variant, the sprite is already there, you just have to change around colors and you're done. The bridge bight be a little harder to make but it could be possible.
2. karts / sprint cars
This one is more so a wish than a suggestion. Nearly every major Motorsport is in this game except those two. If you cant make both, id rather have the sprint cars since theres not really a dirt-focused car.
3. Pit road
The main thing is game is missing is pit road. Theres no way for the cars to change tires or refuel, not that that matters, but it would be nice for the cars to go down in the middle of the race and refuel
Thanks for the work you put into this game (and reading my long review)!