So, Mastodon is working on a communities feature. I think having Mastodon and Lemmy communities have interop (along with great interop between the "same community" on two different instances) would be the superior option.

As an example, if someone posted or tooted the same link to several communities I'm following, I should be able to see all of the comments aggregated/tab between the community posts and responses. In other words, it should aggregate a view of all the different discussions about that link for all the communities I'm subscribed to and/or that my instance knows about.