I think this would have to start off as some kind of defined standard in ActivityPub before it could be implemented across all ActivityPub services and be interoperable.
Otherwise, it will result in fragmentation.
But yeah, having one ActivityPub account for many services is ideal IMO, along with being able to follow any content on your platform of choice (like interacting with Peertube or Mastodon postings in Lemmy, or even something rediculous like interacting with Lemmy posts in PeerTube)