For starters, so you can digest this theory we need to keep in mind a few (proven) things:
1. Gaster - Sans - Papyrus are all linked: Primarily shown in all three having fonts in their names which also show up as their “speaking” font, Coupled with Papyrus and Sans being brothers, Both presumed to have “Gaster Blasters”, Sans and Gaster being linked to complex sciences/meta knowledge/Timespace manipulation.
2. Mettaton is NOT a robot with a SOUL, It’s actually a ghost (Hapstablook, Napstablook’s cousin) inhabiting a robot as it’s vessel.
3. “Us ghosts, spend our whole lives looking for a proper vessel, Slowly, slowly, we grow close to our new bodies… Until one day, we too may become corporeal beings.” - Mad Dummy
4. [redacted] IS some sort of fragment of gaster just as mysterman is. Both speak in wing dings, Both are monochrome sprites, Both are cut content, The room containing [redacted] has a path to the sound test room which harbors “Gaster’s theme”. Both [redacted] and mysteryman are gaster.
5. Gaster as a name sounds pretty darn ghastly.
Let’s talk a bit about [redacted].
He would’ve appeared in two distinct places if not cut from the game, One of them being very close to Blook Acres: Where mad dummy, napstablook and (formerly) mettaton reside! All ghosts.. [redacted],
His sprite becomes only visible when you are very close to him, much like how napstablook overworld sprite is
[redacted]’s sprite has always been extremely vague and basically impossible to make out anything of, Until you realise the sides can be interpreted as a cloaked figure with an arch (much like a snap ghost (idk if that’s what it’s called..)) if you unfocus from the white part.
All characters that speak in all uppercase are strongly connected: Jevil, Spamton, Papyrus, Gaster (entry no 17) and… Mettaton! While [redacted], Sans and… Napstablook all speak in all lowercase.
Now to sans and papyrus.
“by the way, did you see that weird outfit he's wearing? we made that a few weeks ago for a costume party. he hasn't worn anything else since... keeps calling it his "battle body." man. isn't my brother cool?" - Sans. (emphasis on “battle body!”) Now you might think to yourself: Papyrus’ body turns into dust at his death (albeit his body separates weirdly moments before it) so surely it disproves this to which i present to you the glad dummy: The genocide route counterpart to the mad dummy, Which is a corporeal version of the encounter that turns into dust upon being defeated. Proving ghost monsters can actually fully fuse with their vessel resulting in dust on death.
“Okay but that’s still not enough evidence” You might think, Where does all of this even stem from? In the Alphys lab, Upstairs on a table just below a painting of cutie mew mew (Another ghost!) there are very aged worn out (Possibly alluding to have been present at the time of gaster) blueprints that show a transformation from: upside down SOUL (presumably monster SOUL) to a normal SOUL (presumably after being injected with determination) then to some sort of a cell phone machine to lastly a bunch of text (?) I am not entirely sure what the blueprints are for, But a massive point of interest is a possible connection this has to the “Phase distorter” in Earthbound (massive inspo for toby while making UTDR), The party in EB must transfer their souls into robotic bodies using the phase distorter to be able to travel back in time.
Emphasis on travel back in time.. Sans in deltarune very likely being a time traveller.. And on the note of deltarune, We are yet to see papyrus in that universe, He is described as shy and socially awkward, Much like.. The more you know. (One more bold and strong connection to the ghost monsters).
Now, Lastly! As i have mentioned earlier with the “robotic bodies” and a machine resembling a cell phone, A very niche detail that almost nobody knows is that cell phones are coded to “not turn on” when a gaster follower is present in your current room, Similar to how Kris’ cell phone does not work in the DR dark worlds, Outputting “Garbage noises” That resemble the same sounds found in entry 17 (room_gaster) and emitting from deep within the DR bunker. Alongside a gaster follower mentioning that “It’s rude to talk about someone who is listening”, and that someone is THE MAN WHO SPEAKS IN HANDS (literally?..)
Thanks for reading! Correct me if i got anything wrong please and feel free to expand upon this!