Every superhero in the Big two is (partially) a gimmick that needs to be marketed. With names like "Kid Flash, Wonder Girl, Aqualad," mini-versions of their mentors is precisely what they were. And that's ok because they were sidekicks at the time. I don't think they need to stay like that now, but DC needs to allow them to be something else. I can't see how, without most of their history and stuck in bland stories where the JL babysits them and not wearing very distinctive costumes, it's hard not to view them as mini versions of five bigger characters.
BUT:
- at one point, Dick as Nightwing was distinct from Batman because Bruce was a less capable of leader and mostly stuck to the shadows as a grim detective. Dick wore a flashier costume, was at the forefront of a prolific superhero team, and his name was inspired by Superman.
- at one point, Donna was heavily tied to the Titans of myth, a corner of Greek myth Diana rarely interacts with. She has been the goddess of the moon. She has had the ability to create photonic energy blasts and shields, and project three dimensional images of people's memories. And some of her Troia outfits were distinct from Diana, not to mention her supporting casts in Man's World like Terry Long (believe it or not, Donna was better with Terry than she is now), her adopted family and the aliens related to New Chronus.
- Roy we covered. Wally sidestepped all this BS by actually taking over his mentor's role.
- Garth always had the aquatic thing going on, but his first Tempest costume designed by Jimenez made him stand out from other Atlanteans whereas his current one makes him look like just one more face in the Atlantis crowd. That combined with the spell casting made him distinct from Arthur.