Anki, imo, already has an open algorithm (that the user can change via plugins), universal interfaces, and is "self-hosted". My eyes perked up at REST api, but it doesn't look like there's a centralized server that hosts shared cards, which is where my mind went.

I'm building https://github.com/AlexErrant/Pentive/ which is basically Anki + Github + Reddit; people can optionally upload their cards for others to download/fork, and the most popular cards rise to the top. It's FLOSS, offline-first, supports plugins and p2p syncing, and is very much a WIP. My proof of concept is almost done though, which demos the critical technologies in a secure way.