I know it already exists in the US (or at least i think it does). But a bank account where i can get paid in AUD, and spend in AUD, but I save in BTC - and the exchange between BTC/AUD happens instantaneously every time i get paid or spend money.

There is also a quick feature which allows me to swtich my BTC balance from Lightning to on chain once I've saved up a decent enough stack and the mempool is clear.