First off, there are some small things that could be polished in the menus:
The Parts Shop button prompt to return to the "start race" screen could say "back" rather than "cancel," indicating that your changes will be saved rather than discarded.
Button prompts showing that you can change your car color on the vehicle select screen would be helpful.
The "start race" screen in Grand Prix could say, for example "Ruby Cup, Act 2 of 5" rather than just "Act 1," to give an indication of your progress.
This is a bigger change, but in-game, I think it would be helpful if the "slipstream" indicator were made to stand out a bit more. As it is now, it feels like it requires me to take my eyes off the road to see if it's filling up. Rather than grey text in the corner, maybe it could be moved to the center top, made in a bolder color, or both?
Another medium-sized change could be made to the way the arcade mode ends as you complete the course; as the game is now, it feels a bit anti-climactic the way the screen fades out, and very similar to when you run out of time. The first time I completed arcade mode I was actually confused because I thought I might have run out of time at the last second. It would feel much more satisfying if there were a unique screen effect of some sort indicating that you've completed the course, rather than the white fade-out. Ideally, I can imagine a finish line similar to OutRun or HangOn, but I realize this would be a significantly labor intensive addition.
As for actual gameplay changes, my main suggestion is that the "cooldown" for the slipstream meter could be made longer. In other words, in the current game, if you don't fill it up completely, the rate at which it empties out back to zero seems frustratingly fast at times. The fact that it doesn't immediately deplete to zero makes if feel like you should be able to continue progress filling it between drafting multiple cars, but this seems to almost never happen. I think making it easier to fill the bar between multiple drafts would make the game feel more rewarding. This also stems from the fact that one of the main challenges in the game is timing when to swerve out from behind a car you're drafting before rear-ending them. I think it would decrease frustration in a good way if players felt more safe in swerving away from cars ahead of them without losing their progress in gaining a slipstream bonus.
Finally, I have one more long-shot suggestion, maybe for a sequel if not an update: being able to build up a slipstream boost bonus you could store and use when you need it. This would take the game a bit farther from its 16-bit game design roots, but I think adding one more reward mechanic like this would go a long way to increasing replay-ability and raising the skill ceiling. My ideas for how the player could build up an extra boost are:
-filling a boost bonus meter when you slipstream while drifting, since this is significantly more difficult than slipstreaming on straightaways
-filling a boost bonus meter for holding a slipstream for an extended time, or chaining it between passing multiple cars
The meter could fill up slowly, and perhaps fill up to three spaces which could let you store three on-demand slipstream boosts at a time. Perhaps filling up all three could have a special bonus, like letting you smash cars off the road when you use them all at once, and this could be another way to defeat rivals!
I know I was getting pretty elaborate there at the end, but I enjoyed the game so much couldn't stop thinking about how it could be even better, because I think the idea and the base game are so good they deserve to be all they can be. I don't know if this was the best place to post this little write-up so that people see it, but it seemed likely that the dev might see it here since the steam page is more cluttered. If you read this, Ansdor, thank you for your attention and thank you for an excellent game!