The mentors that tempt them into taking a darker path.
You know what I never got ? The whole concept of Nightwing having different mentor/potential father figures besides Batman and playing with the idea that they might force him to switch sides. There was Deathstroke (*1), the short lived Owlman bit in Forever Evil and Raptor. Now I haven't read the last one but these don't ring true to me because Dick Grayson is the golden child. He can do no wrong and is the standard set for everyone else. He's the leader, beloved big brother and has countless people who constantly praise him.
It worked with Damian and the first Nobody because Bruce was having a rough patch with him.
I'd say it sort of worked for Tim during the Ra's Al Ghul heir bit (*2) because he was isolated from the family and you could kind of buy him seeing some good in what he could get out of it. I don't think it ever went far enough in terms of Tim being tempted by the scope of the power to say whether it worked for the character one way or another. But there are certain perceptives that don't fit characters and Dick isn't the outcast so the shady fatherly mentor doesn't click for me. There's nothing for him to gain from such a relationship.
*1 I believe it was during the Renegade thing although he's always been too interest in Nightwing.
*2 This was also during a period where Bruce was considered dead.