The Infocom archive is really good for this https://archive.org/details/infocomcabinet.

The Deus Ex design doc was released http://www.gamasutra.com/view/news/285520/Annotated_version_... along with some other in development Ionstorm games that never shipped.

Nintendo released some early design sketches for classic games like Super Mario Bros and Zelda which are interesting to look at. https://www.nintendo.co.uk/News/2016/December/Take-a-look-be....