It does indeed sound amazing. I wanted to go even further and implement my own activitypub based features on my webserver, in order to make it behave exactly like I want, while still being able to interact with Lemmy and Mastodon, how cool would that be?
But then I stopped, considering the privacy considerations. I've spent years removing myself from the web, from a time where we were considering that being "highly googlable" was cool, and then I came back with perfectly sillo'd identities, only using thematic forums (or alike) and never linking my profiles with one another, so that I can't be profiled easily. Putting all my social activity in one place where one can just GET /outbox on a server known to belong to me to get all my activities sounds quite a step backward from that. I'm still considering it, though, because I really want to be in control of my data. But I need to figure a way to do it that would not be a regression in privacy.