In terms of monetization, I think you could consider optional ads. By default, they could be off, and those who want to support you could opt in. I also don’t know if you have considered a donations page. Not sure how many people would be willing to donate (I wouldn’t, unfortunately), but at least giving people the option might be worthwhile. Regarding game features, I think being able to play on past tracks could be interesting. Custom liveries would also be cool in my opinion. Also, I think being able to see the full leaderboard would be preferable for me.