For example, every 10,000 blocks, the new block would propose an opening balance for all addresses that are still being mentioned in the blockchain with non-zero unspent outputs. It would be an opening-balance transaction.
All validating nodes must then recompute the opening-balance transaction and verify that it is correct, after which all previous transactions in the blockchain can be deleted/forgotten. It would effectively be a new genesis block.
It should allow validating nodes to free a lot of disk space and permit nodes with less capacity to participate in the network.
Is there a reason that prevents us from introducing this?