A lightning payment or p2pk cashu token behind proof of work is an interesting concept that could provide an incentive to "expire" a message 

 It doesn’t need to be timestamped 

 The other day I was reading about ring signatures. I don’t know if they work for Nostr private keys, but if they do, we can use two private keys—the sender’s and a random one—in a ring signature. Then we can reveal an arbitrary part of the random private key in the “ephemeral message” so we can control how difficult it is to guess it. 

 The thing with time-stamping is you can't prevent someone else from time-stamping your data. If someone can prove that the signature existed prior to when someone else could have cracked the key, that's evidence that the signature wasn't faked. 

 You guys are much smarter than I am, but this was my best effort to express what I’d like to see in Nostr.
 
https://github.com/nostr-protocol/nips/pull/1595

cc: @fiatjaf @jb55 (that will probably my first and only NIP, I promise)