Lemmy search already is quite excellent… at least here on lemm.ee, we don’t have many communities but tons of users subscribed to probably about everything on the lemmyverse so the servers have it all.
It might be interesting to team up with something like YaCy: Instances could operate as YaCy peers for everything they have. That is, integrate a p2p search protocol into ActivityPub itself so that also smaller instances can find everything. Ordinary YaCy instances, doing mostly web crawling, can in turn use posts here as interesting starting points.