Cool, this is somewhat similar to an idea I had before: two persons each make a list, and the app informs both parties of common items of the list without disclosing the rest. That would solve a few of the same use cases you mention, and help decide e.g. what movie to watch, even if one of the parties (e.g. my SO) is somewhat ashamed of their preferences (e.g. romantic comedies), unless the other person shares them.

Not sure if this is interesting to anyone, but in any case, I'm unlikely to be going to make it, so feel free to run with it.