A new FEP that will need to be written in the future, depending on how standardization goes: OAuth2 Rich Authorization Requests (aka “OAuth2 dynamic scopes”). It is a possible solution for the category of problem exemplified "@firstname.lastname@example.org visits bar.example.com and wants to boost a post they’re viewing on bar.example.com". That particular example may be contrived, so I wouldn’t get stuck on it specifically, there’s a lot other interactions one could dream up where this is helpful.
Where the FEP comes in is: these Rich Authorizations are going to need… you guessed it… community-managed ontology/understanding.
Then, I’m sure lots more FEPs will come about as different software projects want to provide peers’ users a near-first-party kind of experiences, where it makes sense, if it makes sense to (as compatibility is once again showing it is the foundational problem).
Once thing that I would especially like to see is full migration capability across the Fediverse (or at least between all platforms where it makes sense).
Because Fediverse aims at empowering users, enabling them to choose their home instance and change it, I feel an additional step would be to enable users to test and change platforms, moving from Pleroma to Mastodon, to Misskey, and back.
It seems to me (but maybe I’m being naive), that such a possibility is not a major hurdle and would “simply” require:
to decide upon a list of platforms that can be made interoperable (Masto, Pleroma, Friendica, Hubzilla, Zap, Pixelfed… others?)
come up with a list of common features that exist on all platforms (username, bio, followers, follow, block…) and agree on a standard import/export format
implement a generic moved-to/moved-from mechanism that builds on existing implementations in Mastodon, Pleroma, and Zap (at least I know they provide something along those lines)
To the best of my knowledge, this does not currently work though there have been some effort within the Pleroma community but I think that was for full servers (see this issue), though I heard about people investigating single user migration, I did not find any information about that.
Yes, the idea of a advisory flag is not a new one, it was discussed on the thread you linked. it has a major problem though: If I (on cybre.space) reply to Gargron’s “followers replies only” post (on mastodon.social) and send it to all of my followers, and he doesn’t allow my reply, all of my followers on mastodon.online, glitch.social, and every other instance will still display it, right underneath Gargron’s post (because they have no way of knowing whether I’m allowed to reply or not).
From an anti-harrassment perspective, this is regarded by many, many users we’ve talked to as worse then not having the setting at all.
A software I know of that gets this right is Diaspora, but they do this by requiring that all replies are federated out the instance of the original post, rather then the post author
In Smithereen, I do it the same way as Diaspora. I only send the reply to the authors of the top-level post and the reply that’s being replied to, if any. The server of the author of the top-level post is then supposed to forward it to everyone who might have the thread.