...ok... so just tried to hash the entire .bitcoin/blocks on my node... that takes a long time lol. Something like: (following) might work better/ be more practical...

sha256(sha256( nonce + NormalValidBlockData + hash2(hash2(first n bytes of all previous blocks/blockhashes concatenated + nonce)) ))

where n is difficulty2...

come to think of it... there really is no need for difficulty2... difficulty1 (the normal difficulty) will have to come way down after the block interval goes way up due to 1) asic deprecation and 2) more complex hash function...

effectively putting the big hash in the inner loop of the little hash.

This would be very difficult to build an asic for... specialized hardware is fast ram. a commodity.

incentives to keep blockchain size down. way easier to verify than to hash... but it breaks node pruning.

Could be very simple to include the hash2(hash2()) value... just encode it in a transaction in the block. Nodes check to see if the transaction exists by calculating the hash per the reported nonce and looking at OP_RETURN or something... if not... the block is invalid.

...another idea to prevent an initial shock to the block creation time is to initialize n as a function of the regular difficulty such that n = 1 at the current difficulty (on flag day).