Back when we were discussing site design, @Jin came up with this design question and we do have some answers which could be relevant to our 404 page.
I think we need to get something defined, as ours are very bland.
Bicycles have this for their 404:
Which I think is wonderful.
We are using an SEI standard Captcha page (HAL) and we have the laptop/token Error page but we need a 404.