Since some large instances do not relay public posts, one option is to leverage the rss feeds they provide for hashtags.
For smaller instances, being able to follow or subscribe or have those posts automatically added to the local instance’s public time-line (either local or Fediverse) would allow users of those smaller instances to be able to search across a broader range of instances and see posts and topics easier.
For smaller instances, themes are easier to coalesce around and having some popular tags visible on the local instance from the larger or other instances in general without having to hope they are relaying or somehow connected to the local instance makes participation and engagement across the fediverse much easier and a more pleasant experience.
I understand that RSS may be a touchy subject considering that it’s being used by many right now to just cross-post from Twitter, etc. I also know there are bots that can be set up to do something similar.
In my mind, this would look something like an extra line in the Relays settings where the type of “relay” would be rss and then the posts would “federate” to the local instance’s timelines more-or-less ‘as-intact-as-possible’.
I know for me, one hurdle to convincing some friends to join the fediverse is the difficulty in finding some content that is more readily available on the larger Mastodon instances. Relays help for sure, but having the ability to focus the TWKN feed around topics and interests shared by instance members would dramatically improve the new-user experience by lowering the hurdle to finding new content they are interested in. “The Algorithm” is good at what it does and for many, having to work extra hard from the very first step into the Fediverse just to see anything interesting already makes a painful process for new users even more frustrating.