How about making it possible to run a mailing list from a Deltachat client?
Mailing lists are a federating protocol still in widespread use, mostly in the tech community (they are also more common in Germany). They just have zero PR budget.
Mailing lists were initially just an enormous BCC list that let you send a e-mail to lots of people, but then an automated tool was added to make that easier.
A public mailing list is one which anyone can read, but posts have to be approved (they may only be permitted from list subscribers, or fron whitelisted posters, or they may need manual pre-approval, etc.). The e-mail addresses of posters (but not subscribers) are typically exposed, which sometimes causes spam (surprisingly little, actually); Deltachat’s disposible addresses could mitigate this.
The exposure of poster e-mail addresses means that members can PM posters, or respond publicly on the list. Lurkers can only be contacted by the admin.