If the problem is that no one will post in an open thread near the end of it's lifetime, a silly solution would be to automatically create an open thread every day with 10% probability. Now someone considering posting has no reason to wait for a future day to post because the expected lifetime of a thread is always constant, and is ~10 days.

Better would be to have a single thread "Open Thread" where posts older than N days would be moved to an automatically created "Open Thread Date-Date" post.

An even easier fix would be to remove the end date from open thread titles. When someone feels like posting a new one they just do that. This is a sloppy implementation of the first solution.