Inspired by a friend getting a random email and it sparking a memory for me: https://pageday.org, a global message lottery where each day a random message is drawn to be the homepage.
Yeah that's a good question, and something we haven't codified.
Odds are that we'll curate quite heavily to keep it interesting and maybe along similar guidelines to hn with "anything that gratifies one's intellectual curiosity" rather than just "anything".