Cross-game collectible sounds neat! The idea of looting/achieving something in one game which unlocks a new experience in another game would be fun to explore.
Cross-game collectible is certainly a neat idea, but fairly hard to execute, and will tamper with each game’s own economics. It could be something that does not interfere with in-game performance, such as skins. It’s something that requires significant partnership effort though, and requires some initial strategy to gain traction. Would be interesting to explore for sure. Imagine a world where gamers crave for certain skins like how people offline crave for collectible coins and stuff!