There's no doubt that Wikipedia is a giant store of information. And yes, they have an API, but the data isn't exactly structured. But that didn't stop Jim Blackler, who created Quizipedia, a fun game based off of Wikipedia entries.
Google has long fielded complaints of sign-in requirements associated with Google Play Games. The Google+ requirement and multiple sign-ins required for multiple games has been a hotbed of frustration. Google announced the end of both requirements as part of a Google Play Games API model change.