All currency would be private property, so there would be no “national currency,” if you mean currency that is state controlled/printed.
In my mind, all forms of “currency” are simply promissory notes that are capable of being drawn upon a tangible asset. These notes may be traded, and their value would be relative to the demand of the asset backed by the note. So, in this sense, the currency aspect has value only so far as it is used as an instrument to expedite trades. Instead of sending a man twenty gold bricks so a man can buy a horse, a banker can send that man a note that is worth twenty gold bricks (via a contractual promise which the holder of the note can drawn upon the issuer), which that man can then trade the note for the horse. The horse-seller, knowing he has a note that is worth twenty gold bricks, also does not have to obtain the twenty gold bricks to trade - he can simply hand the note off to someone else who would value the currency (ie the promise of 20 gold bricks) or who wants the gold themselves.
So, anyone can print currency, backed by any asset. You could have a plutonium dollar or a wheat dollar, but you, the issuer, by creating the note, are obligated to provide that asset to the holder of the note when he wants it, whomever it may be.
It is up to the traders in a given market to determine the value of different types of currency for themselves.
The role of the police and the courts, in this system, is to protect the rights of the note-issuers, by fighting things such as counterfeit, and protect the rights of the note-holders, by forcing issuers to honor the promise for payment made when they printed the currency.
Furthermore, to print currency for an asset you might not have at the moment is not necessarily fraud (hence why we can have interest on banking), but you are still obligated to honor the note. To print currency and then refuse to honor the holder, is fraud.
That’s my take on currency.