It's so hard to start from nothing, but your concept sounds deligjtful. The tarot card idea is really cool—anything to bring up new questions and throw new ideas into the mix.

For the folkloric elements and worldbuilding in general, my favorite way to weave it in is to find a touchpoint for character or theme—something that could be echoed in the worldbuilding. So if the outside story is about war and connection, I could add worldbuilding elements with a strange affinity for each other that, if processed a certain way, can be made into a potent weapon.

It's a bunch harder when you're working with existing folklore, though, since you're working within the scope of traditions and stories that already exist and can't be changed. But you've got this!