I'm going to start gathering the findings on Saturn NiGHTS into Dreams here.
The "Nightmaren eggs" first discovered in the remaster are present in both the PCOM and RD_P files of the Saturn version, these files being for common objects and for each Nightmaren's specific level. Crewle and Pole's egg is still missing.
Several textures related to the "blue door" object are present in the E3 beta's PCOM file, and KetrinDarkDragon made a mockup of the palette as the intended palette appears to be missing.
An old sprite of Wizeman's hand was discovered, as well as Wizeman's final hand textures, which led to Ketrin discovering that the rumored mirror staff belonging to Selph was in fact a misinterpretation of Wizeman's fingers all along.
Jackle's original tarot cards are present in RD5M.
Among the sprites related to Clawz' boss fight is an unused spikeball and a longer spike with a similar design to the pillars visible beneath the large gear in his stage.
Frozen Bell's textures include a seemingly unused sign reading "Under Construction."
NiGHTS turns out to have two alternate, streaked versions of the normal cuff texture which are used while drill dashing.
NiGHTS also has 3 extra unused faces in file PLTEX, which appear to be 1. a mimic of NiGHTS' infamous silly face from the opening fmv, 2. a wide toothy grin, and 3. a face with puffed cheeks, probably meant to be used with the "big head" model found in the remaster.
Some of Selph/Self's boss nameplates were found among other bosses' names.
The yellow and blue orbs on the corners of yellow hoops were to have two extra color schemes (yellow and red and yellow and green) and a different animation upon destruction.
A mysterious purple sprite with a rotating animation was found near other collectable item sprites in PCOM. We speculate that this may have been a life-up for NiGHTS.
Two mysterious gold, blue, and green textures were found in the E3 version's PCOM file.
A large and likely unused sprite of the word RANKING was also found.
The snow in the E3 version of Frozen Bell had a different palette than the final.
Twin Seeds' starting platform was to have a palm tree to fill the final's empty tree planter. It is unknown why the tree was removed. The level was also meant to include two additional buildings which are based on some of the buildings visible in the cutscenes showing Twin Seeds. GigglyMan made a mockup of how the tree was meant to appear in-game.
The E3 beta has some kind of unused title screen featuring an early version Spring Valley's background and has a slightly different design for the "Night Over" message.
Reala's drill dash model was discovered to be used in the Saturn version's two-player mode.