> but it would be trivial to let people transfer in cosmetic items (think stickers, color schemes, etc.) based on another ecosystem/game/achievement/etc.

It would not. Every one of those items would need to be intentionally added by the game devs. Every piece would need to be created, modeled, textured, and animated.

I suppose it would be trivial if you created a pile of uninteresting and uninspiring games with the exact same engine, assets, and art style, like so many NFT's of depressed-looking monkeys