We don't know much about the spin-off just yet but I thought I'd give you a list of some of the villains I'd like to see on the show. This does not guarantee that the characters mentioned in this article will actually appear on the show.
10. The Bug-Eyed Bandit
This villain is set to show up in a Flash/Arrow crossover featuring Ray Palmer aka The A.T.O.M. That can't be a coincidence. Bertram (Brie on the show) Larvan developed technology to control insects and uses them to commit robberies. The Bandit knows Ray's secret identity but that knowledge is complicated because he keeps losing his memory. On the show, she will be a female hacker with tiny robot bees. I believe the "All-Star Team Up" episode will not be the last we see of her.
I've always been fascinated by this Superman villain. It seems extremely unlikely we will ever see him in the DC Cinematic Universe but he could totally show up on the show. Rudolph Jones was a janitor at STAR labs until he was exposed to a radiating substance that transformed him into the life energy sucking organism known as Parasite. With the STAR labs connection, he might start off as a meta-human on the Flash and then switch over to the spin-off.
What would this show be without the Atom's arch-nemesis? David Clinton was an ordinary criminal who learnt the appreciation of time. After being released from prison, he created several time-related gadgets and returned to his career as a thief now with time working in his favour. This is one of the more sillier villains and seems to share some parallels with the Clock King who might also appear on the show. Maybe Clinton will be his sidekick and turn against him later in the series.
7. Doctor Sivana
A major villain for the Captain Marvel family, Doctor Thaddeus Sivana would make an interesting addition to the show. He is a mad scientist in the likes of Hugo Strange. If he doesn't show up on the show, perhaps Strange will.
6. Solomon Grundy
Cyrus Gold has already showed on the Arrow as a Mirakuru fuelled soldier of Deathstroke, he seems unlikely of ever showing up on Arrow again so therefore, he would make the perfect edition on the show. After prolonged exposure to the chemicals while he was dead, Cyrus will be resurrected as the mindless invincible zombie known as Solomon Grundy. He will prove to be a tough villain for the superhero team.
5. Killer Frost
Now I'm not suggesting that it will be Caitlin that turns out to be Killer Frost. I have doubts that she will ever be Killer Frost. There are lots of other people who have taken the mantle of Killer Frost and taken on Firestorm. Perhaps the New 52 iteration, Louise Lincoln. Either way, she will make a great foe for Firestorm and the rest of the team.
I don't believe Brainiac being a villain in a Justice League film will do this character justice. Making him one of the big villains in a tv show would show us a great version of the super intelligent, Coluan conqueror. He feels more like an end game villain so don't have him show up right away.
3. Lex Luthor
Is Lex Luthor too major of a player in the DCCU to be in this tv series? Maybe, but I think this character would bring in an interesting dynamic to the show. He has quite a bit in common with Ray Palmer, he is a businessman, he owns a company and has built his own armour. He feels like a great villain for early on the series.
2. Vandal Savage
This guy has one of the more interesting back stories. He was a caveman until he was exposed to a meteor that gave him heightened intelligence and immortality. I loved him in Justice League: Doom, it showcased just how powerful this man is. He has the resources and the know-how to take anyone he wants down. He will be a very formidable foe for the team.
1. Injustice League
This is everything that this list has been building up to. Okay, this isn't necessarily a villain but what team up show would be without a team of villains as their arch-nemesis? They don't have to be called Injustice League, maybe they can be the Legion Of Doom. Either way, we need to have a team of villains as the big bad on the show, it makes sense.