@pioneer I totally agree, this would be a huge game changer. I think with some sort of single sign-on, a lot of people would see a huge value-add in Fediverse.
If you haven't seen, this conference panel from last year has Christopher Lemmer Webber talking about protocols already in ActivityPub that could be used for this. Starts at (24:00):
https://archive.fosdem.org/2019/schedule/event/activitypub_panel/