Thank you so much for your words, it means so much to us!!
To answer your questions: we also feel like the story we wanted to tell is pretty much completed, and we don't plan to expand on that.
What we do plan to expand a little bit on, also answers your other question: no, currently the black and golden cards are just fun-to-see easter eggs, we didn't got the time to further explore them, but we are planning to implement some kind of side-puzzle for them - something that will not be necessary to finish the game, but will unveil some cool secrets :)