I mean sure, if you want me to go into all of the "Theory-Bait Lore Mystery Shenanigans", I'll just make a list, but I doubt it would really be productive.
Sans:
To start with the obvious.
Sans-"that expression you're wearing...that's the expression of someone who died twice in a row."
Etc.. Nothing to be added here we all know the run down.
Sans-"our reports showed a massive anomaly in the time-space continuum. timelines jumping left and right, stopping and starting. until suddenly, everything ends." "i always thought the anomaly was doing this because they were unhappy. and when they got what they wanted, they would stop all this. and maybe all they needed was...i dunno. some good food, some bad laughs, some nice friends." Same as above.
"look, i gave up trying to go back a long time ago. and getting to the surface doesn't really appeal anymore, either. or maybe that's just a poor excuse for being really lazy? hell if i know. all i know is, seeing what comes next...i can't afford not to care anymore."
Been discussed to hell and back already but like yeah, Sans wanted to go "back" somewhere (or possibly some "when"), and he specifically differentiates this from the surface. Even in the Pacifist Ending he seems to utterly not care about being on the surface. He mentions that he has since given up on trying to go back, however. Meaning obviously he attempted to do something and failed.
Sans-"wait a second. that look on your face while i was talking. you've already heard my spiel, haven't you? i suspected something like this. you're always acting like you know what's going to happen. like you've seen it all before. i kind of have a secret codeword that only i know. so i know if someone tells it to me...they'll have to be a time traveller. crazy, right? anyway, here it is... (whisper) (whisper) i'm counting on you to come back here and tell me that. see you later."
Etc..
Secret Room;
"(There's a photo album inside the drawer. There are photos of Sans with a lot of people you don't recognize. He looks happy.)
Note; The dialogue specifies that the photos are "of Sans", specifying that the other people are "with" him. Meaning that he is the center focus of the photos.
"(You look in the drawer.) (There's some kind of badge.)"
Obviously implying he had a badge to designate him as some kind of scientist.
"(Blueprints.) (You can't read the symbols they're written in...) (...or maybe it's just the hand writing.) (Looks like they relate to some kind of strange machine.)"
On one hand, Wind-Ding's would commonly be described as symbols by the average person, and well, they are. On the other hand, Alphys is explicitly said to have terrible "chicken-scratch" handwriting. It's one of them that helped Sans make the machine, I mean shit they literally gave him the blueprints for it. It's most likely Alphys as the game says "or maybe it's just the hand writing".
"(There's a strange machine behind the curtain.) (it seems to be broken.)"
Obviously referring to the machine in the blueprints. Toby is very careful with choosing his words.
Don't even get me started on the image of 3 people with the words "Don't forget" written on it, which literally confirms that Sans has some significance to Deltarune, if nothing else.
Papyrus-"I ALWAYS JUMP OVER THE GAP INSTEAD OF SOLVING THE PUZZLE. SANS NEVER SOLVES IT EITHER. HE ALWAYS JUST... UM... GETS ACROSS. I THINK HE HAS A SHORTCUT OR SOMETHING."
Btw this is a potentially good Papyrus feat. He jumps over a pretty massive gap of many tens of meters in one bound, and he does it consistently, like it's easy for him, might be worth looking into? I'll make note of it.
Undyne often refers to Sans as just Papyrus' brother, implying that she doesn't really know him despite him being close to Alphys and, well, Papyrus.
Papyrus-"YOU'RE IN MY HOUSE. GOOD CHOICE! THOUGH I GUESS IT'S TECHNICALLY SANS'S HOUSE TOO. BUT I PREFER NOT TO DISCUSS HIS PART OF IT. HIS ROOM IS... IT'S LIKE ANOTHER WORLD! A WORLD WHERE THEY DON'T KNOW HOW TO VACUUM. YOU'RE BEST STAYING AWAY FROM THAT STRANGE PLACE."
No further comment needed.
One of the puzzles in Snowdin is a crossword puzzle with a drawing of the mascot for ICE-E's on it, a character never mentioned in Undertale even once but shown directly in Deltarune, as the mascot of a pizza place literally down the street from Sans' (Deltarune) very house, and Sans probably would love pizza given his personality and taste for junk food.
Undyne-"This is Papyrus's brother's station. I'm always catching him slacking off, or sleeping... But his results are JUST good enough to not fire him. Well, except today. He didn't tell me ANYTHING about you."
Minor but noteworthy.
Papyrus-"HUH? SANS ISN'T PLAYING WITH HIS TELESCOPE? WOWIE!!! MAYBE HE WENT TO WORK!!! WOW!!! AM I ALLOWED TO BE PROUD???"
Alt-"HUH? MY BROTHER? OF COURSE HE HAS A TELESCOPE. SANS LOVES OUTER SPACEY SCI-FI STUFF. HMM? HE NEVER TOLD YOU?? YEAH, SANS NEVER TELLS ANYBODY ANYTHING!"
Papyrus-"A LAB??? MY BROTHER WOULD LOVE THAT! HE LOVES SCIENCE FICTION!! ESPECIALLY WHEN IT'S REAL."
Also, inconsistent as Papyrus didn't know what a laboratory is in another instance and didn't seem to learn what it is, either. Smh Toby plot holes. But anyways ye Sans scientist stuff.
River Person-"Tra la la. Beware the man who came from another world."
In addition to;
"Don't snoop behind people's houses. You might be mistaken for a trash-can."
Probably unintentional but Sans and the word trash are very commonly related to each other, not just by Flowey but by Papyrus as well, and the only house that has a back room is the skeleton house, so River Person is probably talking about that.
"Beware the man who speaks in hands."
Don't need to comment on this everyone and their mother knows the significance of this to another character.
"The Angel is coming." and "Did you ever hear the old song coming from the sea."
Both very likely references to Deltarune.
Sans (Deltarune)-"(Winking) especially considering I've never met you before."
The door used for Sans' room is identical to the doors used as teleportation fast-travel stations, or rather, shortcuts, in Deltarune. Considering that Sans definitely didn't construct the doors used in the Dark World's, a lot could be said on this.
You get the picture. Could list more but long enough already.
Alphys:
Papyrus-"HMMM... A MYSTERIOUS SCIENCE HOUSE."
Sans-"the sign outside says "lab.""
Papyrus-"LAB?"
Sans-"like... laboratory."
Papyrus-"LABRADOR... Y? DOES THAT MEAN THERE ARE DOGS INSIDE?"
Sans-"i mean. (Winks) i wouldn't rule it out."
The fact that Alphys is perfectly fine with Sans very blatantly knowing about the Amalgamates, means she clearly has immense trust in and respect for him.
I forgot to mention this earlier but Alphys LITERALLY says that she has researched alternative timelines and KNOWS they exist. She mentions it during Mettaton's quiz, which obviously ties in with the whole "our reports" thing. Skip to 1:55
Among the multitude of camera's Alphys has, two of them are literally right next to areas that Sans is usually at, including his god damn SENTRY STATION. Implying that she is probably especially concerned for his well being, as it's a stretch to say she's spying on him, let alone that he wouldn't notice and be bothered by it.
Undyne-"Oh man!!! We're gonna be able to watch anime online?!"
Alphys-"Of course, Undyne! What do you think we've been fighting for all along?"
Sans-"yeah, what do you-. whoops."
Alphys-"Jinx! I knew you were gonna make that joke!"
Papyrus-"WAIT. ALPHYS. I DIDN'T KNOW YOU KNEW SANS!"
Alphys (profusely sweating)-"W-Well, I...-"
Sans-"doesn't everybody?"
RinneItachi-"HMMMMMMMMM NO IMPLICATIONS HERE. CLEARLY ALPHYS BEING NERVOUS AT THE MERE MENTION OF THEIR RELATIONSHIP, AND SANS IMMEDIATELY COVERING FOR HER TO DILUTE THE SITUATION, IS IRRELEVANT!!!"
"Papyrus-OH-HO!!! THE PUZZLE THAT DR. ALPHYS MADE! I HAD TO ASK UNDYNE TO ASK HER TO MAKE ONE. I ONLY FOLLOW HER ONLINE... I'M NOT REALLY FRIENDS WITH HER... YET!!!!"
So Alphys is good friends with Sans but doesn't really know Papyrus at all, the most important person to Sans by a mile. Clearly not significant!
So basically Alphys and Sans have a deep and old friendship.
Gaster:
I mean most of this is common knowledge or just not significant to the thread. Probably a skeleton for a multitude of reasons. Related to Sans in some way, objectively speaking. The former Royal Scientist, the creator of the CORE and presumably the True Lab. Was working with two people at the creation of his experiment mentioned in Entry Number Seventeen (not Entry Number "17" keep in mind, that one was made by Alphys wink wink). Those two people could be anyone, but the most likely candidates are Sans and Alphys, or Sans and Papyrus. However, it's a pretty known fact that Papyrus knows literally nothing about the weird science shenanigans going on with his brother, so I would guess that it's Sans and Alphys, who went on to literally take over his position as the Royal Scientist anyways. Keep in mind it's not confirmed that anyone who knew Gaster in the Undertale timeline forgot about him. It's implied that it might be the case, but the idea only came from Goner Kid talking about how the world wouldn't change at all without your presence in it, how, even if you never existed, nothing would really be different in the grand scheme of things, which people obviously think could be Gaster speaking through Monster Kid, or just an alternate version of Monster Kid.
Anyways I'm getting a little sidetracked. Basically Sans MOST LIKELY worked alongside scientists that were 100% Extraordinary Genius, in a group of many scientists, possibly and likely including the Gaster Followers we meet around Hotland's. We 99% know that Sans is a timeline-traveller, more specifically that he came from Deltarune to Undertale and tried to go back (failed), and has an in-depth, complex understanding of the inner-workings of time and space, however at the end of the day he failed to apply that knowledge in the creation of a machine that could send him through timelines again, even with the help of other scientists, including Alphys and/or possibly Gaster, which is sadly an anti-feat against Extraordinary Genius, but it wouldn't rule out a "possibly" rating if things went that way. He is extremely analytical, psychologically tuned, and accurately predictive to a superhuman extent. It is suggested that he has sci-fi level knowledge about complex concepts such as time and space, and was at least close to the level of scientists who would be considered Extraordinary Genius on the wiki, especially Alphys. Sans is aware of the game mechanics and is the only character to abuse them in his fight without hesitation, and he might be aware that the world is a game considering that he very blatantly breaks the 4th wall on numerous occasions, though he is kind of a gag character in some ways which could explain it.
Again, as I said before, going by actual feats and whatnot, "At least Gifted, likely Genius" is the most "objectively accurate" rating, however if we include implications and all that, it's easy to get Sans to "At least Genius, possibly Extraordinary Genius".
