dansup, the most curious question — does this work across instances, or does it only consider local posts? Recommendation systems are "easy" when you have access to everything you could possibly recommend and you can do all kinds of queries against that data. Not so easy when you have no idea most content exists in the first place.
I'll go another, equally uncharted way though. I want to try decentralized search by centralized identifiers. You'd import your friend list from somewhere, like Twitter or VKontakte, and then some ✨magic✨ will happen and you'll see fediverse accounts associated with those centralized identities. It should, in principle, be possible to make a DHT (distributed hash table) out of instances, and in that you'd store somehow preprocessed centralized IDs as keys and fediverse IDs as values. The idea is that you should be able to find a user on any instance from any instance.