There are tons of great 404 page designs out there. However, most applications ignore this important page. It’s a chance to show creativity and humor. Don’t be so serious. We only get one life, maybe you should try to make people smile a bit more.
Here is the one I designed for the Marketo application 404.
I am an agent of creativity and humor. Not everyone is good at it, but most people appreciate it when they experience it.