This idea has been suggested recently in this discussion, under the name "related posts". (I've just changed it to "Related Discussions" because that's what we're calling "threads" in Flarum.) If you have any concrete ideas about how this feature might work, please feel free to add them there.
Reinhard It is very necessary to have this feature in the core of Flarum.
As @Digital pointed out in the discussion I linked, this would probably be better done as an extension. This is because not all Flarum sites will need this feature, and there is undoubtedly more than one way to implement it; some Flarum admins may simply prefer a different implementation. Features that need to be optional and/or replaceable don't really belong in the core.
Generally speaking, it's best to avoid specifying "core" in a feature request because that's only asking other users (many of whom don't really understand the distinction between core features and extensions) to pile on with their opinion of whether it belongs in the core or not ... when we really want to be talking about how the feature should look and function. ?
(If you're curious about the difference between core features, bundled extensions, and third-party extensions, you should check out the introduction to extension building.)