By this, I mean genuinely ANYTHING. You could even say something like charfoal in umamusume and I think at least three people would agree. For me, I personally would love to see a Puppet Master Artful skin for Die of Death. His music box could be a keystone, his wall could be some sort of shadow magic, so on and so forth.
A close second would be Flora and Florian appearing as cameos in Block Tales. If Zuka from Phighting can be in this then I don’t see why they can’t be there too. (If we want to get hopeful I’d say Slurpy in Cooking Mama. Maybe)