I've been thinking about how great this feature would be. I don't know how Lemmy's codebase could handle it, but this just makes sense to me and would solve the big issue of community fragmentation.
In general the idea is that when you open the comments section of a given post, it should fetch all other comments from crossposts (ones with the same link). Since Lemmy already keeps track of crossposts by linking to their threads, it's just a matter of fetching those comments and showing them in conjunction with the comments in this community. When you reply to a comment that's federated from another community, your reply will go over there.
Also, from what I understand, copies of posts and comments are stored in the instance's local database, so it shouldn't be very expensive to fetch these comments from different posts, right?
How viable would something like this be? Are the Lemmy devs considering something like this?
If lemmy stays small, very. On reddit, I liked being able to discuss it inside the boundaries of one community or the other. But reddit was so huge, splitting things up was good. Here, it's bad. So yes it's a nice idea.
it'd make sense if it was optional for the person making a crosspost.
sometimes you'd want the crosspost and original to be linked because the audiences are similar (say, crossposting a speedrun for a game to both that games community and the speedrun community), and sometimes you'd want them to be separated (say, crossposting something to a BestOf style community)