This is a simple and small idea, but not that it necessarily could/should be implemented.
Related to the prevention of quantum attack on the cryptography, on some hypothetical situation where all of the private keys of the entire history are calculated and published, which would render the entire history useless.

Let Bitcoin ("original" Bitcoin) be forked twice, into Bitcoin-A and Bitcoin-B.
Bitcoin-A operates mostly identically to Bitcoin-original, so it is sort of it's "successor".
Bitcoin-B will be the watchdog, it will use a more (likely) quantum-resistant crypto for the ownership/spending.
When someone spends on Bitcoin-A, that person will also have to replicate/translate that transaction on Bitcoin-B. When blocks of A are being formed, only transactions that are duplicated in B are allowed in A. And a markle tree root (or something) from Bitcoin B's "block" must be present in Bitcoin A's block - and ofc validators from A would have to "read data" from B to function.
Then initially, Bitcoin A is the source of truth, and Bitcoin B follows that truth arbitrarily.

Let's say one day all of the keys from Bitcoin A are exposed. So Bitcoin A's history goes to the trashbin, but the users can, say, rollback a few days and switch into Bitcoin B, and then B would now operate independently from (the now defunct) A - assuming B survives that attack. This could also help to inhibit the attack in the first place, since there would be a backup around.

What you think?