Yeah I'm pretty interested in this and Topics is only the first iteration of the idea. At the time piefed.social was basically a single-user instance so making the topics admin-manually-curated groupings was fine. Perhaps it's time to take things to the next level, tho.
The concept of 'multireddits' sounds like a bundle of related functionality and I'm not sure which parts of that you're interested in that PieFed's "topics" doesn't already do. Is it the user-created part? The subscribing part? The join-many-communties-at-once part? PieFed does some of those already but you won't see it unless you're logged in...