Recently cult of the dead cow came out with veilid, a secure decentralised way of sending encrypted information and i think this could be a huge asset to lemmy. Cult of the dead cow even mentions mastadon by name as they say this is the type of application it was designed for. If veilidchat takes off then i really hope the devs will consider it.
Plus, ActivityPub is server-to-server, it’s one instance pulling data from another. I guess what you mean could be a user accessing their instance but like you say, Tor already covers that.
It could be more relevant for something like the… name escapes me… the instagram fediverse site. Or well some file sharing system of course.