There have been some requests to make it possible for Mastodon users to create threads on Lemmy. At the moment this is not possible because Mastodon cant set the title field which is mandatory in Lemmy. The same problem affects other projects like Pleroma or Friendica (where title is optional).
It is unlikely that Mastodon would make any change in this regard, so a workaround in Lemmy would be necessary. I implemented such a workaround which makes the title optional when receiving a federated post, and uses the first line of the content instead. This workaround could also be made configurable per instance or per community.
But to be honest I am not sure the workaround is a good idea in general, as it could result in bad titles, and generally more low-quality posts. Thats why I want to know what the community thinks about it.
So you have the same approach of generating a title from the post body. Only you do it on the frontend and not the backend. In Lemmy that might be a bit complicated because the body is already optional, and we would have to somehow encode that either title or body must be set (or both).