I like this idea, but I would suggest generalizing it to apply to any similar posts. That way, we can find the best answers to a question without having to open multiple posts asking the same thing.

Issue #1908: Merge comments from similar posts into a single view

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.

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.

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.