This would increase server load and traffic by a huge amount though, and would the benefit really be that big?

There are already quite good places to find communities for stuff you’re interested in, and less niche stuff will be federated “automatically” by these rules unless you’re on a small instance.

Good community lists:
https://lemmyverse.net/communities
https://browse.feddit.de/

Reddit communities now on Lemmy:
https://sub.rehab
https://redditmigration.com/