Part of the reason svirfneblin are so boring is because nobody can agree on what a gnome is, including D&D. That's not true, actually, but the one thing we all do agree a gnome is - a squirrel-sized do-gooder voiced by Tom Bosley - is not what RPG/fantasy gnomes are allowed to be. Instead, gnomes occupy the valley of the shadow of dwarves and elves, picking up whatever traits from each are omitted from any particular characterization, to the point where I literally just made the gnome race the product of aaaaages of elf-dwarf fucking. D&D goes back and forth on this, usually having multiple types of gnome at a time, and that's currently codified in the 5e PHB: you can be a dwarf gnome (an underground gem-loving tinkerer) or an elf gnome (a magical waffle who talks, walks, and squawks with the animals, do).
When you get down to making the goth underdark version of elves you can make them an effective "black mirror" to crib a descriptor from Gaiman. They are beautiful and graceful, but not like an elf, like Ridley Scott's Alien. They have truck with the creatures of this world and the next, but not stags and talking eagles and wisps. Spiders. Eyeless, formless aboleths. The damned or buried, a.k.a. next door neighbors. You can cover a lot of ground with it. When you try to make a "black mirror" for dwarves, surly chauvinistic xenophobes who toil endlessly underground and are defined by violence and greed, you've a harder time of it. D&D has traditionally responsed by giving them crazy magic powers (which I love) or setting them on fire (which I don't). So if it's an uphill skate with dwarves and gnomes are just spare-parts-dwarves then doing anything interesting with svirfneblin at all is going to be next to impossible and then D&D does the worst thing they could possibly do with them, which is to make them GENERALLY PLEASANT AND UNAGGRESSIVE and their power is literally to be ignored.
Do you know why snifflebitches have endured? Because they, like the salad bar at Whole Foods, are sourced local. They are a place where you can have a cuppa and a shave before getting back to the adventure, maybe hire some retainers or sell some enchanted amber, and get on with the actual fun part of the adventure, the reason you are in a tunnel system so deep that normal subterranean races don't even get to come to the party to begin with. When you are in the bowels of the earth, walking to actual Hell or Hell's PO Box, they are always around. And so they have always been around.
Now, I find that lame. It's one thing to see something lame in this hobby but when the answer to why they're still lame is "They've always been lame" then that's lamer.
You can do amazing stuff with these guys, obviously, by basically taking the fact that the only thing they have going for them is a name that sounds like a sinus infection and dumping just a pile of good ideas on it. I really could stop this article right here and say "Just do everything +Patrick Stuart says about these guys or duergar or derro" but I won't. (Seriously though, why wouldn't you just ignore these guys in favor of derro? They're techhhhnically related to dwarves not gnomes? Fuck that, when the source material paints that as a meaningful distinction THEN I'll pretend to care. Besides, even if you never get to any of the D&D lore about derro or anything that Patrick wrote for them, they are still murderous psychos from the lost continent of Lemuria who definitely caused WWII and the Holocaust and their old school picture looks like Karg from the He-Man movie. Which isn't just more interesting than sirfverbeefle, it's more interesting than the DROW.)
Instead I decided to look at what they can do, what makes them a unique gnome subrace, and....I liked what I saw? I was surprised, too. A score bonus, an extension of Darkvision (120' is enormous, that's 36 1/2 meters, pace that off and tell me that doubling their Darkvision isn't a huge gain), and a Stealth trick. I do feel like the Stealth trick just keys off their "ha HA I look like a rock if I don't do anything, rocks and inaction are awesome!" schtick, but I like the idea of the trick. It was thinking over these subrace qualities that I decided what I needed to do with these guys, first, was to approach the gnome, and, second, approach these guys as a subrace who live deep deep within the world only incidentally, because that was the only place they could find a comfortable dark on a regular basis.
Gnomes are Pygmalion as political cartoon. A forest gnome is a dwarf's idea of everything stupid about an elf: skittish little rabbit-fondlers whose magical tricks are at best cute distractions and at worst useless. A tinker gnome is an elf's idea of a dwarf, busy with some useless little toy that'll just break and is no substitute for real magic anyway, obsessed with THINGS instead of life, instead of the world quite literally around them in their uncomfortable, cramped homes. These racist caricatures evidenced themselves in the art of their respective cultures over the centuries, tapestries, filigree, statuary. Then came Garl Glittergold, shining-eyed god of brotherhood and mischief. If he could unite dwarf and elf with a common brother then the world would be a better place, and if he failed then he would unite them against a common enemy, so same end result.
Garl Glittergold kissed the breath of life into the carvings of enemies and made gnomes.
The plan worked tremendously. After a period of suspicion and persecution elf and dwarf settled into calling the gnome Friend. After all, elves found they actually did have a lot in common with forest gnomes after all, and their vision of tinker gnomes was still, at the end of the day, an elf's type of dwarf. The inverse was true for dwarves, concerning the different manner of gnomes. It actually succeeded in giving elf and dwarf an excuse to find common ground. Over the centuries, however, divisions grew once again, because the only ones who seemed unable to tolerate tinker gnomes were forest gnomes and vice versa.
The rise of man and man's fear of everything provided the solution. Y'see, gnomes are wrong. They look wrong, they move wrong, like in a horror movie when they film something backwards and run it forwards. They have faces that don't quite work or correlate to standard expressions, like chimpanzees. And since all gnomes enjoy a prankish sense of fun, they were constantly playing with the humans they encountered during the ascent of man, whether the humans liked it or not. A different view of gnomes emerged, of hairy little creeps who always lurk in the darkness, beautiful only in the way arthropods are beautiful, and the literal things of nightmares.
Obeying the rules of stories and sympathetic magic more than anything else, Garl Glittergold did it again.
Midnight Nomes are hirsute as a Mexican circus werewolf. Their heads and clever hands are distended, like a goblin's. In the harsh light of day, torch, or full moon, their hair has a dreamy dandelion property like those fiber-optic light toys. It is a good thing that they're so hairy, because their skin, eyes, and other organs all have a slight translucence to them. Nothing approaching a magical status or a gelatinous cube effect, more like a Visible Man. But a nome. They wear lots of clothes, layers, sweating buckets from creepy cloaks and hoods, anywhere bright light might be found. It doesn't hurt them but it makes them itch, makes them nauseated, and makes them self-conscious. Midnight Nomes abhor the sight of one another in the light of day, and are appalled at their own reflections.
They prefer the night, and shadows, and grow bolder and roam wider as the month and moon wane. Midnighters are invisible in the unlight of the True New Moon, the period of middle darkness.
The nights are far too short, however, and so the Midnight Nomes went down, past the forges of the dwarves, past the shimmering gem caverns of their tinkering brethren, down to where a liquid dark suspended every individual molecule in a solution of naught. They were comfortable there, where their skin melded with the black and their bodies became nearly imperceptible. They walk looking up, where on the surface they always walked looking down, still careful not to meet one another's gaze. They built modest little villages, and began to venture surfaceward only at True New Moon, for food and supplies and trade and mischief, a constant ascent/descent trek to their little hovels. Underdwellers and explorers found the Midnight Nomes gracious and hospitable because they found villages where only the young, weak, or old dwelled, too infirm at least to make the trip up and back. The helpless have nothing to offer even animosity except welcome. In this manner, through these tales, did stories of the Midnight Nomes' generosity spread, endearing them to both man and gnome, while becoming a new kind of nuisance for elf and dwarf.
It never ends.
Superior Darkvision: You have 120' Darkvision.
Jellyflesh of the Midnighter: You have Advantage on Stealth checks made at night, and are +5 to Stealth in total darkness. You are invisible for four hours from midnight during the True New Moon.
This actually incorporates a form of a Garl-gnome-creation-myth I found on wikipedia, keeps my view of gnome as midpoint between dwarf and elf, gives them their own weird Wonder Woman origin, explains why they're so different while so similar to existing kinds,and makes the deep gnome in particular governed by weird motivations and tensions and gives them a character beyond existing. I did ditch the ability score bonus they normally get in favor of augmenting their Stealth trick and making it more conditionally robust.
It'd also give them a good reason to have a hate-on for all the other Underdark races, since these would be preying on only the Midnighters' most vulnerable and convenient, which is monstrous.