Artemis is actually adapted from the villain Artemis Crock. (Because the only other DC characters named Artemis are a redheaded Amazon and the Greek goddess.)
Six of the seven leaders of the Light being Vandal Savage, Lex Luthor, Ra's al Ghul, Queen Bee, Ocean Master, and the Brain. What no one expected was Klarion being the seventh one. This one was helped by the fact that before the show premiered, it was already known the late Miguel Ferrer was Vandal Savage and Marina Sirtis was voicing Queen Bee, Oded Fehr and Mark Rolston already voicing L2 and L3 respectively before we saw Ra's and Lex on-screen, L5 being voiced by Roger Craig Smith (who also voiced Orm), and L6 having a French accent like Brain (although this involved The Other Darrin, Nolan North was the one initially doing the voice, but it changed to Corey Burton by the time he physically appeared).
Cheshire being Artemis's older sister. (Not in the comics.)
Also, Sportsmaster being Artemis's father. (As in the comics.)
Sphere being the Super-Cycle from the Young Justice comics (also, it and the Forever People's Super-Cycle are the same character here).
M'gann's personality being lifted from a TV series called Hello, Megan!. (Hinted several times.)
M'gann being a White Martian. (As in the comics. Hinted even more than the above.)
Guardian being Red Arrow's uncle. (As in the comics.)
Sort of. He's actually another clone of Roy Harper, programmed to think he's Roy's uncle.
Prince Orm being Ocean Master as revealed in tie-in comic issue #14 (As in the main comics.)
Conner being the half-human hybrid clone of Superman and Lex Luthor. (As in the comics, though a Retcon made after the Young Justice comic series which included him.)
Starro being the tentacle organism found in Atlantis and being involved in the Light's plot. (Even though it wasn't named, nothing else fits.)
Impressively, some predicted that the term Sportsmaster used to describe Red Arrow, "Broken Arrow", in the episode "Targets" would be significant in the future. It was: "Broken Arrow" is a shut-down command implemented into Roy (The Mole) to retrieve information or to program him even further.
The fact that Season 2 would start with a time skip.
And once an image of Nightwing was leaked, some people guessed a more specific number, namely 5.
Aqualad quitting the Team and joining Black Manta, and the reason for which being Tula's death. (Because of leaked images of toy packaging.)
G. Gordon Godfrey is Apokoliptian (or at least is working for Darkseid). (As in the comics.)
Granny Goodness (another one of Darkseid's elite) being the founder and head of Goode World Studios.
The Goode Goggles being a brainwashing machine due to their resemblance to one of Granny's devices on Apokolips that are seen in most depictions.
Jason Todd is revived by Ra's al Ghul. You'd be hard-pressed to find a fan that didn't expect that at some point.
Doom Patrol existed/will exist.
Violet being the spirit of a Mother Box possessing a human body.
Terra is with Deathstroke. (As in the comics.)
Two-in-one. Jace was secretly a spy/traitor (as in the comics, although she worked for the Manhunters in the comics, and the Ultra-Humanite in the show), and was lying to Violet about her terminal diagnosis.
Some guessed that Dick not being a metahuman was the reason the X-Pit had a worse effect on him than Black Lightning.
Many guessed Granny's machine first shown in "Influence" would be used to spread the Anti-Life Equation.
Infinty Inc. is Lex Luthor's, not the traditional group associated with the JSA.
Terra has a Heel–Face Turn, survives, and joins the heroes properly.
The Legion of Superheroes will join the fray due to being the only other major teen hero group that hadn't shown up yet.
Conner is alive, trapped in another dimension.
Said dimension is ultimately revealed to be the Phantom Zone.
Season 4's episode titles were hinted to be building towards the phrase "Invitation to Kneel Before Zod", and ultimately, Zod himself finally appears in the Phantom Zone.
