Based on the show's history and the adaption of each DC arrowverse show, there is always going to have original characters appearing throughout the season. But there is an issue when this happens:
- Arrow show seems to love Felicity so much by the showrunner that Marc wants to get rid of Laurel Lance so that Oliver and Felicity could be together as a couple. A slap to the green arrow fans who want to see the 'OliverxLaurel' a thing.
- The Flash where Barry Allen normally could solve the crime with the help of his friends or defeat his enemy by himself. But lately, he too depends on Team Flash to solve it. And now supporting characters can get power-upgrade so easily.
- DC' Legends of Tomorrow used to consist of C-Listed Superheroes like Atom, Firestorm, Rip Hunter and Heatwave running the team. But as the season goes on, more established heroes are leaving the time stream and more new original characters are flowing in until you can't tell if this show is DC Comics series or just a normal Time-Travel adventure with random people.
Honestly, I don't know about this but yes, some have existed before in the comics but some of them are a duplicate of the original characters just the title name is the same (Ex: Dinah Lance, Kelly Olsen)