I've found something very interesting on the website of Tony C. Giovanni - one of the Level Designers for Retro Studios during production of the Metroid Prime games.That link wasn't working when I tried it, but there's more concept art and extensive excerpts here. Via.
Its a proposal document for "Metroid 1.5" (working title) a rejected Metroid game set to take place between Metroid Prime and Metroid Prime 2: Echoes. It seems Tony was tasked to write it up following some internal meetings at Retro, and is nearly 7000 words long. Its fascinating!
*Previously: Metroid paper toy.
*Buy Metroid toys at eBay.