hashtags?

Personally I'd find it pretty useful but it does depend on the user.

A Post Flair system would be great, specially for keeping things organized in a community that also serves support issues, for example having preset tags for "Issue", "Bug", "Solved", "Discussion", "Announcement"...

I'll say though, it is important that those can be edited/swapped my moderators.

These help keep posts organized and helps filtering for users that come later.

What we have to do now is use [TAGS/LIKE/THIS] and use external Userscripts that add the ability to filter those, which isn't ideal.

This is exactly what I was hoping for, you wouldn't need a multi-reddit and it's implemented smoothly. This post shows exactly why I was hoping for it. I usually have to go directly to Fediverse to see anything and I didn't think there would be a follow up so fast, so I didn't go directly to it. Meaning, that's why I'm 4 days late to this post. If I had my own user tags, I would put this community under "favorites" and check it way more often for new posts. Even if it doesn't get implemented, I appreciate you guys considering it.

Anyone even unprivileged users could have the ability to tag any user, post, community or instance. It would be a different set of tags visible only to themselves. The content shown when visiting those tags would be the same as the ones for everyone else.

User specific community tags. Each user has a completely different set of tags, visible only to themselves, that they can create and apply to communities. When a user visits those tags they see the posts from all the communities with that tag. The content shown when visiting those tags would be different as the ones for everyone else.

Consider replacing communities with tags - lobste.rs style. Would eliminate cross-posting and reduce duplication. Could increase participation and improve cross-instance content preservation.