"multireddit" are nice to have but they do not address this problem Which a common view for all user of an entire community across the fediverse. "Multireddit" require client to pick and choose individual communities. This means less than 1% of users will every use it. This means there will never exist a fediverse wide community around topics.

Propagation and agglomeration is a problem for clients not servers. Server only need to propagate a "we have new stuff" message and it's up to the clients to pull it and cache it. In any case, users should be able to click /c/books and see the content of all /book/ on all instances in a single location. Unless most users can do this with one click, there will not exist a fediverse wide community.