Thank you for the thoughtful ideas and feedback!
Having another currency is an interesting idea. We are planning a Prestige currency, which won't be working for the demo, but can be previewed.
This is what's in our current build, so pardon the WIP numbers. 
The way it'll work is that you can choose which global upgrades to spend this on. It'll unlock new stages and affect things like GoldCoin or "time away" earnings. On top of the currency, you get the x# coins and balls to speed through the early levels.
This is the current plan! We're going to try it out. I hope this scratches the itch of having meaningful rewards instead of "do it again, but faster." I'm realizing our game is similar to Egg, Inc. in some ways and I don't like the same-ness of every level in that game.
This week we're trying to finish the new demo build that shows off some improvements and changes. You might hate some of them! But your feedback is very welcome, and we'd love to hear it. It's been really helpful to hear from the itch community and we do take it all in and discuss everything!
If the maxed out Auto/Drop/Split feel really bad, I will tweak or remove it. My hope is to have bursts that can vastly exceed the base amount, but I notice that the bursts don't feel meaningful when the screen is at a constant fever pitch already. Definitely let us know if you hate it when the new build is up.
Again, thank you so much for the feedback and for taking the time to read this! We're excited to wrap up the new demo. It'll be shorter this time. Our steam page is up if you want to follow there too: https://store.steampowered.com/app/3684530/PlinkIdle/