This is particularly tricky question...well done!
My first instinct is to say: I WANT A WITCH CLASS! Problem #1: I realize you asked for a subclass and not a class. #2: The Witch class concept, myth, literature, and folklore are too vastly broad to boil down into a single subclass. #3: 5e is overrun with spell casting and casters.
Next thing that comes to mind: I WANT AN ARCANE HALF-CASTER!: Problem #1: In 5e, while 1/3rd casters are self-contained enough to work as a subclass (Take a fighter, add a little magic/few spells. Take a thief, add a little magic/few spells. Simple/single concept.) HALF casters in 5e, the Paladin and Ranger, on the other hand, are broader in concept ensconcing multiple subclasses of their own. They possess their own -albeit limited and largely derivative- spell lists. #2: 5e is overrun with spell casting and casters.
Then I think: PSYCHIC SUBCLASSES FOR EVERYBODY! Problem #1: You asked for ONE subclass. #2: 5e is overrun with spell casting (and I fully expect 5e's Mystics will function, basically and regardless of fluff, as spell casting) and casters (which psychics essentially are, however one wants to view "psychic powers" in the fluff or mechanics of the game).
SOOOO I'm brought to either Fighter, Thief, Ranger, or Barbarian needs another NON-MAGIC-USING SUBCLASS to at least begin to bring some form of parity in the class structure of 5e.
OF those...hmm...SWASHBUCKLER's the first thing that comes to mind. Could be a Rogue subclass. Could be a non-spell using RANGER! Guy to roams the rivers and seas! But #1: It's already done/in there somewhere (how good/bad/over-/under-powered anyone thinks it is is a separate conversation). #2: More broadly on the thought, a non-spell using ranger subclass has already been done, as well (its good/bad/over-/under-powered design is another separate conversation).
Next I think, WHY are ALL of the Barbarian subclasses, except the "core/default" one, magical shamany spirit-using guys?! What else do we have from myth, literature, and folklore for a Barbarian do aside from or in addition to "Raging" and "Spirit/Totem Magic" on which to hang a subclass concept...ummmm....well, there's...hm...a "dex barbarian?" Any clamoring for that?...Anyone? No? hm...
Naturally, turning to the oft under-served Fighters, one's mind immediately goes to the Class that Dare Not Speak Its Name. Then I think, YES! Obviously! #1: It ticks the "from a past edition" box. check. #2: It's not supposed to be a spellcaster. check...?...but if it's NOT a spell caster and gets anything resembling its previous edition features, if 6 or so years of ENworld threads have taught me anything, people are going to start losing their sh-ields. And if it IS a spell caster with superhuman/paranormal capabilities, a large chunk of its fans are going to cry foul and start losing their shields that it's not "right" or "enough" like the class that dare not speak its name they want....So, really, this is a lose-lose for the designers and the players...let's keep going.
So I end up back at the thief and go to, give us a useful and flavorful ACROBAT! #1. It has pedigree gravitas from the 80's cartoon and (even before the publication of) 1e's Unearthed Arcana. check. #2. Separate and apart from gravitas, it gets "nostalgia points" because its not been seen as an official thing SINCE 1e (unless maybe it in one of 2e's "kit" books? idk). check check. #3: NOT a spellcaster! check.
Is it the one subclass I desperately WANT? No. It is a class concept I don't already have homebrewed and like/use just fine? No. Is it the ultimate idea of something I would want to add to D&D if I called the shots? Almost certainly not.
But does it satisfactorily answer the thread question of, "What Subclass would I add to D&D if I could?" Yes.