Create a social network. User identities are created through proof of work. Messages are permanent, restricted in length, and posting is rate-limited per identity.
All of this is to prevent making the gimmick too easy to use. But you don’t tell anyone yet.
Get a bunch of people on, stir up discussion, encourage shitposts. Do what you must to generate a good amount of organic content. If you can get multi-lingual users on, even better. Let this continue on for a while. At an arbitrary point in time, maybe after a milestone amount of messages, flip the switch.
All initial restrictions on identity, message length and volume are lifted. But users can no longer free-form compose messages. Instead, every word (or sequence of words) must be a pointer to a word (or sequence) in an existing message. Pointing to pointers is fair game, for convenience’s sake. Vocabulary on the network is now set in stone, forever unchanging. How will this transform the use of language? What kind of content will people create?