I think countries need laws on the duration of classification that get to the heart of national security vs public interest. I think a core issue is that classified information should come out within the lifetime of people involved with very few exceptions. Additionally, crimes found after declassification should have automatic statute of limitation renewals.
One system that may work could be automatic mandatory release unless renewed at a higher level of classification. Every time material is elevated it should require the highest level of administration to approve renewal. Additionally, when things are declassified they should be publicly advertised in some way so that records requests can find them.
There is a need for classified materials, but there should be a high bar to make them and a low bar to release them in order to avoid the government using them as a shield to hid activities they don't want the public to see.