Some Lemmy compatible platforms do stuff like that

• allow you to see both Lemmy style content and Microblog style content in the same platform
• allow you to follow individual users.
• include hashtags that federate out so the Lemmy posts appear under that hashtag for microblog users

Something like that could work, as long as it's an additional option and not a change to how communities currently work.

Mixing hashtags with communities sounds like a bad idea because of how much content would come in at once. For example, there are a few RSS feed communities and already those ones are overwhelming to keep up with. Most posts sit with no comments and 1 vote, which doesn't work for the vote & comment based way we organize content here.

We can improve how we federate these platforms together, but I prefer the tagging method. That way it's a conscious decision to post a microblog post in a community.