The opposite of liabiliteis, i.e. debt and own equity, is assets. The net of outstanding financial assets and debtliabilities is by definition zero as one party's financial asset is by definition the counterparty's debt. The net of libilities and assets in a balance sheet is also by definition zero. A government can keep assets for instance in a sovereign wealth fund. One could here mention Norway which has collected oil revenue in a govenment owned wealth fund and which by the same time runs a very small public debt.
The opposite of public debt would be public assets in so far we don't expect it to be any equity in the public balances.