Build a simple P2P messaging service for ethereum addresses, accessible through web3 browser (or similar).
I open the messenger within Trust or Metamask and web3 recognises my ethereum address - it allows me to send messages from that address and see messages sent to that address.
Messages can be sent to any address (i.e. if they don't have the messenger then the message appears when / if they ever sign up).
Sender can't see if the message has been read.
if signing is required (i.e. to verify Identity) this should be once per browser / IP to avoid unreasonable cost of use).
[login to view URL] let's you comment on other people's addresses (no verification of who you are). The messenger being prepared here is specifically to allow verified (& private) address to address communication.