Pure blog sites are different from what I have observed. You can likely see about sponsoring a plugin to accomplish sending a summary of posts with other site content news. However a forum’s Subscribe is typically creating an account as it will send out digests and such if configured.
While there is already a lot of features built in. It is meant to be extensible through things like the plugins & theming.
Guest gate theme component encourages ppl to sign up if their reading. The idea though sounds good as well. You could post it as a Contribute > Feature request. However Marketplace likely be your better bet & quicker.
Others may have other ideas to offer on maybe other ways to accomplish what your looking for.