Decentralized encrypted email.
Create a key, identify it by a hash of it, and encrypt all mail sent to the account with the key. Allow it to run on top of regular email using one or more email addresses as an alias, but have the key itself be the identifier.
Client 1 creates a key pair > uploads email address(es)/"aliases " that client controlls (signed with key pair) > client 2 searches for emails based on client 1's key or aliases > client 2 sends email through one or more of the accepted inboxes encrypted with public key > client 1 reads encrypted email.
Basically a modernized version of PGP that also handles identification, and similar to how it's been proposed to change Matrix accounts to in order to make them decentralized.
Kinda funny, I was just writing about archival media this morning. Verbatim makes DVDs & Blue Rays that last ~100 years, and M-DISC makes ones that'll last ~1000 years. And the Verbatim Blu Rays run ~$0.036 per gig.