“Tastemates”: somewhere out there is someone who likes all the same [books/movies/music etc] as you. 

Have an algorithm find that person and surface the things they love that you haven’t discovered yet.

Makes the recommendations more trustworthy.

Crypto/web3. Author of Read Write Own

readwriteown.com

I want to have something like this for restaurant/sights recommendations when travelling.

In theory, you should be able to build a graph like A and B both liked restaurant X in Paris, A liked restaurant Y in Berlin, recommend Y to B when visiting Berlin.