Monero, digital cash. Not really a solution for your open source wallet that supports all the major coins. But if you’re looking strictly at it from a perspective of avoiding the Ponzi schemes, encouraging digital cash, fungible money, untraceable, the closest analog to digital physical cash you can find. Monero. www.getmonero.org
There are a few open source wallets for monero, most of them are open source, keep the data locally.