I do think adding another superhero show to the CW line up with help the ratings for the network. Superhero shows and movies are incredibly popular right now, and are heavily trending. I think viewers are appreciating the artistry and fantasy-aspect of these types of shows now more than ever before.
If the CW were to add another superhero show, they would appeal to a large number of people who truly enjoy superhero showes and films, yet don't find Marvel's work appealing (since I believe the current ones are DC). Especially if they decided to do another female hero, such as Wonder Woman--they could then gain the market of people who like Marvel but are upset at the treatment given to Black Widow.
There are too many movies and shows all featuring superheroes. But there are favorite superheroes and Netflix and Marvel/DC seem to have them in movies and shows. In addition there are wizardry and vampire movies/shows too, so this leads me to think that CW will not benefit by adding another superhero show to its line up.
Adding another superhero show to the line up a CW will not necessarily increase ratings. The show has to have great story lines that will keep the viewer consistently glued to their seats week after week. So it's not just adding another show that will do the trick but making sure that the show content is top notched.