Very cool stuff, it'll be a lot of fun to play with this.

What I can say is that I prefer to make "themed cubes" with specific sets, rarities, etc. It would be nice if there were some filtering options for the recommender so I could say things like "exclude non-rares" or "only include cards from {these sets}".

It's clear some of this theming is naturally happening, since I'm getting a large number of results that do match my cube theme. (In this case, Dominaria-plane sets only.) Presumably other people are on themes that are similar enough my results are being affected. But having these filters would go a long way.

In particular, a good recommender system is very helpful for me, since when I do themed cubes it's HARD to find all the right cards. I know all the "Vintage Cube" cards by heart, but random uncommons from Mirage? I need as much help as I can get for that one!