Its true Darth Vader is indeed overused because he is the most iconic SW character ever. Kylo Ren is a pale copy that never really lived up to him.
Imho, part of his appeal is his design is simple, mysterious, and more inspired than you think. He is the every man that can turn for the worst if we take the wrong decisions. His outfit is clearly inspired by Samurais, knights mixed with a futuristic look but still founded in reality. We have all of this fascination for evil, and there has always been an aura of mystery around him before the prequels.
His relationship with Luke makes it even more interesting because his son represents what he could have been.