I was surprised how well this runs on mobile! Though this is the first game I've ever played on itch.io, so maybe that is normal.
A few suggestions I'd add:
1. Multiplier balls that give bonus points (2x, 3x, 10x) make them different colors or something and add an upgrade that increases the chances of one spawning
2. An accuracy bonus of some sort, maybe a Perfect Shot multiplier or something for hitting the exact center of the goal. This would allow you to add an accuracy upgrade as well.
3. Some sort of prestige mechanic for longevity. Maybe you can cap the number of people or balls on a field and you prestige to a bigger field that can house more people. Or maybe you can run multiple fields at once with different currencies being earned on each. Then you could have some upgrades cost multiple currencies.
Sorry for the super long comment! I just really enjoyed this one and would love to see it improve.