There's more to explore. In particular, using groups of similar products and users to infer recommendations given only very sparse data is powerful but underexp

There's more to explore. In particular, using groups of similar products and users to infer recommendations given only very sparse data is powerful but underexplored in production usage. (Disclosure: I work at a a company which just expanded their product portfolio in this direction. The first few pilot customers show very good results, but there are a lot of aspects and nuances and alternative approaches we haven't had time to try yet.)