Here’s the thing. I’m a mod for a small-time community for a niche interest, !castles@lemm.ee I’m also on Mastodon, and was before my Reddit exodus. I follow #castles as well as a few other related topics on Matsodon, so I get quality toots, such as this: https://mastodon.scot/@McNige/110926238926867959, that I wish I could just crosspost over to my community. Currently, I have to repackage the toot, which isn’t a huge problem, but currently I just drop them a note on Mastodon that their content has been posted elsewhere on the Fediverse. What would be nice is if people who comment on the Lemmy post also get fed into OP’s toot. More sharing, more connection, more activity.
On the flip side, I’ve subscribed to @castles@lemm.ee on my Mastodon instance and, while it’s good to be able to follow posts in feed form, it looks like ass: 
I realize I should try this with Pixelfed, but I haven’t made that leap yet.
I don’t know, am I thinking crazy here? I’d think we’d want everything in the Fediverse soup interoperable in a more seemless way. Is this a feature request or am I missing some way to do this better?
The ActivityPub protocol makes it possible, but most apps only make use of the pieces of ActivityPub that suit them. For example, in Lemmy they lean heavily on the Group actor type as the basis for Communities, which Mastodon doesn’t use at all. Peertube creates content using the Video activity type, while Mastodon only creates content in the form of a Note activity. Lemmy produces a lot of Link activities, which get rendered into a Note when viewed in Mastodon. And so on.
It’s all a work in progress and I’m confident integration will get better over time. The framework is there.
Kbin has limited support for Mastodon posts but it’s UI is still very much focused on the reddit-style functionality.