One option are keyed-verification anonymous credentials like used in WabiSabi. There's one implementation by @lontivero:

GitHub

GitHub - lontivero/Cashabi: An experimental eChash system based on WabiSabi and Nostr

An experimental eChash system based on WabiSabi and Nostr - lontivero/Cashabi

Yup, the any-amount ecash will be based on KVAC also used in WabiSabi and Signal! The mint only needs a single private key, and the entire wallet can be stored in just a single nut. All that while also vastly improving privacy (blinded amounts, like confidential transactions). It's going to be so fucking beautiful 🥹