This was a satisfying game! I really enjoyed the incremental progress, and the wonderfully clinky-clacky coin sounds. I hope you continue this idea, and add in other levels!
Some ideas, I'm sure you've thought of these, but why not encourage discussion in the comments:
-Junk coins, or paper bills.
-Magnet repelled coins, you have to trap them in a corner to pick them up.
-Different board shapes, with zones that make picking up coins difficult.
-Adding coin placement to upgrade tree, starts with junk, then more valuable coins as that path is upgraded.
-Coins that can temporarily in/de-crease the value of nearby coins when picked up.
-Creatures that eat coins, but if they eat enough, they may poop out an ultra-valuable coin.