If lemmy created a community the same name as the username when someone signs up. You could follow a lemmy user and then when your on mastodon (once we’re federated with mastodon) you could see their self posts in their community as a mastodon post and vice versa.
I think one of the goals of the federated decentralized social media is the ability to interface with other mediums. It’s natural for enthusiasts in this community to be excited at the prospects of interoperability. Plus Mastodon is awesomesauce.
I think it would be nice to have the ability to add a mastodon toot into Lemmy and have it display the toot in a nice format and be linked to the toot’s server, but I don’t think replies to it should be standard from Lemmy since there likely could be different conversations occurring on different mediums.
I also think it would be nice to have PeerTube video integrated well into Lemmy but again the replies on Lemmy should not be the same as the replies on the Peertube format. Likely it will be a specific community focused discussion on Lemmy.