One of the things I didn't figure out for playful was how to talk properly about collecting in play. Which I think is at least as important as pretending and equally neglected.
I still haven't figured it out, really, but there's one phenomenon I want to note: Junior Taxonomy. Arthur got massive satisfaction from sorting and resorting his Pokemon cards in new ways, new arrangements, new structures. Sometimes based on an overt taxonomy baked in by the card designers, sometimes in a scheme of his own devising. He's now doing the same with Match Attax. I remember doing this too. I think it's a basic impulse; we're practising ordering the world.
So, I bet designing for varied taxonomies would be as potent as designing for pretending. Even just putting random numbers on the back of things. You don't have to put the meaning in, leave it to be constructed/pretended.