Anyways, one of the best ways of preserving and observing video game prototypical media is collecting footage of them in action on game catalog tapes.
Sega Video Magazine is a good place to start. They're a series of game catalog VHS tapes distributed in Japan by Sega Enterprises that promoted and centered exclusively around Sega's own games and games from other publishers during the Mega Drive and Sega Saturn eras. A lot of these tapes have footage of games that haven't taken their final forms yet, which oftentimes can exhibit strong, interesting differences in the content from the final releases. The spectrum of this can range from minor to extreme.
NiGHTS into Dreams... doesn't really fall on either side, but lies more in the left-side of the middle. The clip below covers the world unveiling of the game that took place inside the Tokyo Prince Hotel on March 27, 1996 in Japan. Following that segment is an on-screen interview with some of the then-most respected developers at Sonic Team within Sega, who were also responsible for Sonic's greatest games ever. Producer and main programmer Yuji Naka, director Naoto Oshima, and co-game designer Takashi Iizuka take screen time to tell us their stories as to how the beginning of development came about. Unsubbed.
Scattered throughout the interview is footage of the game in it's early stages of development, including the first level Spring Valley that has a different level and object layout. Many of the sound effects differentiate from the final and even the music sounds unrefined, particularly The Dragon Made a Loud Scream. Turn on the video annotations and it'll elaborate the specifics to you.
I've said this many times already but Nico Nico Douga is quite the treasure trove of Japanese gaming history, featuring various promotional videos of various games spanning several decades starting from the 80s. I remember that Super Mario Bros. 3 video that was a big hit with proto enthusiasts in 2011 after it was found on NND. Hopefully, someone will be kind and interested to provide a translation of the interview above.