Zatanna the show is definitely not lacking in girl power but you can never have too much. She trained to use magic by her father Zatara, she's made appearances in Smallville and cartoons such as Young Justice and the 90's Batman series.
Superboy I know it's an obvious pick but it would open the superman family and give more avenues for the writers to choose from. Connor Kent definitely has a different methodology for fighting crime. He was cloned from half of superman's DNA and Lex Luthor's human DNA to fill in the gaps of the Kryptonian DNA they couldn't replicate. He's made appearances on Smallville and Young Justice.
Brainiac 5 as some of you may know he is a love interest of super girl he is a member of the legion of superheroes and a descendent of the original Brainiac yeah know the evil dude who opposes superman. He lives in the 31st century and is a member of the Legion Of Superheroes and is key member with a genius level intellect and a genius invento
Stargirl also known as Courtney Whitmore all though this wouldn't be her first time on the TV screen it would be nice to see maybe a hero rivalry/friendship develop. She has been featured in Smallville and Justice League Unlimited. She has a cosmic staff which enables her to fly and shoot energy blasts