Gregory's Server@cogspace
I agree, that a full ActivityPub support is helpful, but we have in some parts and services also differences (see the handling of lists, quoted posts itself, filtering in timelines, markdown/html support,...).
A quoted post is maybe a message, with a URL and a generated card view (by the host instance) and the usage of the existing notifications.
Or a new ActivityPub Action and new notification types.
However, other services should adapt it, anyway.