Donna's not a Wonder Girl anymore. It's one thing for Wonder Girl, a.k.a mini Wonder Woman sidekick, to run around with a lasso and bracelets like her mentor. Donna's an adult now and needs more visual distinction. Visuals are key, especially with these big characters that have larger than life personas and costumes to identify them at a glance. By putting a black haired Amazon warrior at the forefront of this superhero team with a lasso, bracelets, a shield and a sword, you're basically communicating to the audience that this team is not distinct from the JL that has the actual Wonder Woman on it. Same with there being an archer on this team, except this one wears red instead of green. Or an Atlantean that wears blue instead of gold and green.
This is the problem with putting the Fab 5 in particular at the forefront of the adult Titans, at least as they are now. On a basic visual level, they look like a group of lesser Justice League stand ins. Donna wearing something like her star field costume, lacking Amazon weapons Wonder Woman is way more famous for carrying, and having some extra abilities like her original Troia powers would be far better for her. Same with Roy being Arsenal and Garth being Tempest.