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?
This wouldn't work or make sense for cross posts in communities to have discussions specific to that community.
For example I run the !ancientcoincleaning@lemmy.world community. If a post from there is cross-posted in !ancientcoins@lemmy.world their discussions about the coin would be different (focused around numismatic interests as opposed to cleaning focused discussion). It wouldn't make sense for the comments to be merged.