Black Manta is more known for being an Aquaman villain. Fairly recently we saw shark king was brought to life for all the square eyed comic book t.v. show fans.It would be tough to argue that a villain team up with the brawny shark king and the brainy black manta wouldn't make for good t.v. especially if they get flash on the home turf or home ocean.
Mirror Master should be on almost everyone's quintessential flash rogues gallery if not at least in the top 3. As his name not so subtly suggests he uses mirrors against his enemies. He can also use the mirrors as portals talk about all the potential story lines.
Killermoth although an adversary of batman and not the first insect related villian to be on the show. Killermoth was transformed into a moth monster known as charxases i would be particullary interested to see this cgi creature come to life. As you may have intuitively figured out he is letting moths loose on the city.
A double agent for Slade to infiltrate the titans team. The s on the suite stands for Slade. This is her apprentice outfit when she is being mentored by Slade. She can control and manipulate earth. She has a conflict within in her whether to be a hero or a villain an internal struggle like this would make for good t.v.
The flash has strong female character such as Caitlin Snow it would be cool to see the flash team-up no i'm not talking about Supergirl that already happened. I'm talking about a strong and brave warrior from Themyscira it would expand the dc t.v. universe in a new and exciting way and introducing some magic and Greek mythology into the expanding universe. If training with amazonian warriors isn't enough to convince you she's one tough cookie she has held her own in the titans tower with the likes of Nightwing and the other titans she's no push over.