For users of sites other than Stack Overflow, could some version of challenges work on your site?

Many sites could do writing prompts or writing challenges for native and non-native speakers. E.g. Spanish.SE used to have translation golf. Writing.SE had topic challenges. There are probably more I'm unaware of.

There are many sites that already do things like this:

• 65 Words gets you to write 65+ words daily in your target language (write in whatever language you want); Reddit's r/LanguageLearning has a Babylonian Chaos sticky, where you write in whatever language;
• LangCorrect allows you to get corrections on your target-language writing;
• Jianshu (a Chinese social media app) has a kind of streak feature, where you write 300+ characters per day, and you get badges for certain-length streaks (this is where I post nowadays); likewise for a host of subreddits like r/WriteStreakKorean where you submit your Korean writing;
• Reddit's r/WritingPrompt gives you, well, writing prompts.