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?
I think the part that you need to think through carefully is how moderation policies may differ between communities or even instances. And there are a ton of reasons you might want to not subject users by default to comments from another (eg, a community aimed at a topic with "PG" comment moderation mixing with a similar community that is targeted more towards adults).
So, keeping in mind that the "social contract" of allowing community members to remain protected by the community rules that they signed up for should be something that can be preserved.
Not out of reach, though. Opt-in to per community xposts. Filter out by first non-allowed-community comment per comment thread. For sure do-able.
Just important to consider why we have communities in the first place and to consider the different types of stakeholders.
Good point, I can see how something like this could cause some shock especially in political posts. Having it be opt-in and choosing the communities to connect with would definitely be great.