Things get boring once you hit gem upgrades because money becomes irrelevant. There needs to be a way of spending money to improve gem gain, to make it worth spending gems to improve money gain.
Enemies from all waves are worth the same number of kills, so early waves are better than late ones. One fix would be for enemies to give both kills and increacing amounts of gold, with gold costs being rebalanced for this.
All enemies are currently worth the same, but I am working on some content where you get better "materials" (which can be used for new upgrades) if you are at a higher wave.
Regeneration can't go below 0, which is good to know. It's not all that useful, but you can keep it if you fight all of the Assassins before fighting things that get you regen. Previously I took care not to because I was worried negative regen would be bad.
I found a bug, the "highscore" always updates whenever you beat a boss, even in the same run. E.g. you beat a few vampires early, then later go back to fight the rest, it gets set to the time after you beat the last vampire.
The way speed is handled in this game... isn't good. Early on it makes almost no difference (getting 100 points makes you 4% faster) but at the end it's crazy strong (getting 100 points doubles your speed). As a side effect, the game doesn't speed up much at the start. In addition, this means that it is a good long-term investment to not fight enemies that lower your speed for the most part. It makes a small difference early on but later becomes huge.
The relation between max and min speed has issues. You basically need to be able to go at the slowest speed to survive the harder levels, and you need min speed 4 to be able to get max speed 2 safely!
Also, I would suggest always keeping the best car in the race to keep it from ever going backwards like that.
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All enemies are currently worth the same, but I am working on some content where you get better "materials" (which can be used for new upgrades) if you are at a higher wave.