The general idea came from Telegram: Enable Nimiq Account Authentication on the Nimiq forum.
By signing a message, a user can prove they own certain Nimiq Account. By adding text to the message they could also choose or change their username.
The first step of the project could be to create a SSO server that can be self-hosted. Maybe add it to NWM.
Second step would be to create a Discourse Plugin that connects to sso using the existing Discourse functionality.
Just some ideas. Discussion is appreciated.