Tricky, since it seems to be customary to stay silent if you didn't manage to put a game on this.
I started, knowing that tarot is based on the older taroccos card game. Since there already is a game in there (and it's older than the arcana/oracle part), I wanted to explore this particular angle further (also hey, there's already mechanics, right?). Turns out, it's probably the most complex card game of all time (think fizzbin with much more depth than poker) - anyway, after two days wasted on research (and a visit to the city archives, where they keep honest-to-god 500 year old card decks), the only thing I had come up with was the code for shuffling the deck, drawing and rating cards, with no idea that would fit the jam's criteria, besides just implementing the card game.
Anyway - this is the first game jam I see here that had enough criteria to be interesting. The mystery theme also is great thing.
Next time, I guess. :)