The show Titans is the first live-action adaptation of DC's team of teenage superheroes, the Teen Titans. Usually, the team is made up of mostly former sidekicks and often changes from run to run which is something reflected in the show.
Titans season 3 is currently in production and there are several young heroes from DC comics that fans would love to see get added to the team's lineup, many of whom have already been teased and hinted at in the series.
Arsenal
Roy Harper is a skilled archer and the former sidekick of Oliver Queen aka the Green Arrow that often alternates between the codenames of Arsenal and Speedy. He usually uses a variety of trick arrows to aid him in combat but some versions of the character have been outfitted with a cybernetic prosthetic after losing most of his arm.
He was name-dropped by Donna in season 2 as her contact that gave her information on the villain, Shimmer, confirming that he does exist in the Titans universe and could be introduced as an old friend of her and the team in the future.
Robin
So far, the show has introduced two characters who have held the mantle of Robin, both of whom have retired from the role. With Dick Grayson becoming Nightwing in season 2 and Jason Todd leaving the team and set to become Red Hood in season 3, it's time for another character to come and take their place.
While there are many others that have served as Batman's sidekick in the past, the one fans are most likely to see on the show is Tim Drake. Drake is the third Robin and while, like the others, he doesn't have any powers, he is extremely intelligent which makes him a perfect replacement.
Cyborg
Most fans of Titans are familiar with Victor Stone from the comics and some may even be familiar with the version that was introduced on Doom Patrol. It wouldn't be hard for him to cross over to the series since it takes place in the same universe as Titans considering Beast Boy used to live with the Doom Patrol before meeting Rachel and moving into Titans tower.
The fact that the character is already established would be a good thing since the show wouldn't have to worry about delving too deep into his origin and could just provide viewers with a refresher.
Aqualad
In season 2 of the show, fans were introduced to Garth aka Aqualad who was a member of the original Titans and was briefly in a romantic relationship with Donna before killed by Deathstroke. Even though Garth is dead for good, there is a possibility that the second Aqualad from the comics, Kaldur'ahm, could show up to replace him.
In addition to being able to survive underwater, Kaldur has a pair of water bearers that allow him to use his hydrokinesis to create weapons, usually swords, out of water. Another interesting element to his character since he is the son of the supervillain, Black Manta, which makes him a more complex hero.
Wonder Girl
One of the most surprising moments from the series was the death of Donna Troy. Her death could be an opportunity for the show to introduce Cassie Sandsmark and add her to the team.
One of the many children of Zeus. Cassie is the second person to be Wonder Girl. Even though it seems likely that Donna will be coming back thanks to Rachel, Cassie can have her spot for the time being.
Blue Beetle
Jaime Reyes is the second Blue Beetle that gets powers when he discovers an alien scarab that attaches itself to his back and turns him into a living weapon.
He often struggles to maintain his control over the scarab rather than allowing it to control him which would be an interesting thing to explore in the series. If they can't get Cyborg away from Doom Patrol, he's the next best option.
Kid Flash
In the comics, there are two versions of Kid Flash, Wally West and Wallace West, both of whom happen to be cousins. While it's unlikely that both of them would show up in the Titans universe, one or the other of them might.
It would be great to explore such a character's relationship with Dick and, hopefully, he could a layer of humor that the show needs.
Miss Martian
Even though the show is pretty grounded, the current Titans lineup has a couple of alien characters--with Conner being half-Kryptonian and Kory being from Tamaran, so there is no reason they couldn't introduce a third in M'gann M'orzz.
M'gann is from the planet Mars but while she poses and identifies as a Green Martian. She's secretly hiding the fact that she is actually a White Martian. She's already been shown in animated form on Young Justice and in live-action on Supergirl which is where she gained popularity. On that show, she is the adopted sister of Beast Boy who is already a member of the team.
Orphan
Another Batfamily character that would be cool to see is Cassandra Cain. The name might sound familiar considering she was recently in Birds of Prey but, unfortunately, the name is the only thing she really has in common with her comic book counterpart.
In the comics, Cassandra is the daughter of two Batman villains, David Cain and Lady Shiva, that was trained to be the perfect warrior. Due to her father's desire for her first language to be combat, he deprived her of speech, making her illiterate. She later rebels, gets adopted by Batman, and goes on to become Batgirl. Her fighting skills alone would make her a great addition not to mention it would be great to see the character finally done right in live-action. Also, since former-batgirl, Barbara Gordon, is confirmed to be in season 3, she could very easily take on a mentor role to the newest member.
Static
Virgil Hawkins is one of DC's most popular young heroes and was even the star of the animated Static Shock series. Virgil is a young man that gains the power of electrical manipulation after an event known as the Big Bang where police used a chemical named Quantum Juice against civilians. This backstory would be a very relevant one to see discussed meaningfully today.
In addition to his abilities, Virgil is also a skilled inventor and has even designed and created several of his teammate's costumes in the comics. While it has been years since the animated series ended, the fanbase for the character is still alive and well.