Maybe this post would be more at home on a lemmy.ml community, or on github, but this place is my home and I feel like I'd like to get opinions here first :o)
Right now, there is no way to make an instance agnostic link for a post or comment, like you can do with users or communities. Each instance assigns its own number to posts and comments, and there isn't really an easy way to find a post or comment on one instance when starting from another.
For example, if I get a lemmy.world link to a post in a lemmy.world community, in order for me to comment from lemm.ee I would first have to go to lemm.ee/c/community@lemmy.world, then find the post in there manually. If I want to reply to a specific comment, I would have to find the post on lemm.ee in this way, then also search that thread for the comment. If you're looking for an old post with a lot of comments, this can be quite challenging.
Instead, I think lemmy needs to revise how it numbers comments and posts, using the same system used for users and communities. Rather than every instance using a different number, they should use the federated host's number followed by @hostinstance. The local user would still see the original federated link, but users from other instances would see it in their own instance with a tag for the federated host instance. This way, anyone could easily edit any link to make it work in their own instance (provided that link has already been federated in their own instance).
Here is a specific example I wrote in a comment elsewhere:
For a specific example, to me, this post is https://lemm.ee/post/1726780. In the original, federated instance, the post is https://lemmy.ml/post/2308622. Instead, it should appear to me as https://lemm.ee/post/2308622@lemmy.ml.
Your comment, to me, is https://lemm.ee/comment/1409174. The federated link is https://lemmy.one/comment/1393053. This is the link you see, and should continue to see, but I should see https://lemm.ee/comment/1393053@lemmy.one.
Using the instance in the numbering scheme means each federated host instance manages its own numbering while ensuring there will never be a clash between instances. https://lemm.ee/comment/1393053@lemmy.one and https://lemm.ee/comment/1393053 would refer to two completely different comments. This information is already being transmitted in the process of federation, so it’s just a matter of tagging the data when it’s received.