Increment proof of work, every time you use the app, you do some work for yourself and for the people you are talking to

This PoW is a bearer token that can, somewhat, act as a proxy for both being an Agent and having friends (that tried to find PoW for you).

Finally, you can use Bitcoin to prove that you had that key for long time, which spammers can't do.

All of these are tricks I am using in #mns to allow random short name, but they can work with Npubs as well I think.