There are now two forms of currency, instead of just one. Formerly (and in the current itch build), you can only spend coins on upgrades. With our next update, you can also spend balls. Some upgrades are stage based and cost balls, and other upgrades are global and cost coins.Some examples:
Costs Balls, resets each stage
Costs Coins, and investments are permanent
Auto Drops and their cooldowns
Peg upgrades
Multiball Trigger
Manual Drop Cooldown
Ball Rarity
GoldCoin boosts
Ball/Coin multipliers
Disco Ball chance
The wheel will still be there to add a random chance element as an extra reward for achievements and prestige. But progressing through stages will now offer a "choose your reward" moment where you pick from up to 3 buffs. The buffs now have a varied wide variety of rarity (common-to-legendary) as well -- so if you're lucky, you'll get a legendary offering that will boost the game significantly.
We got a lot of feedback that it's sad when there isn't an upgrade for every stage, so now there's an upgrade for every stage! Some are really impactful, and some are just a helpful boost, like making active or passive play stronger over time.
If you want to see these changes in action, there will be an official Steam playtest that you can sign up for here: