Out of the official names in the running, I'm all for Ezra too. Don't understand how the other two could even be considered.
But people still comes up with their own suggestions. I will do so too.
From the category mentioned above:
Zegers (born to play Grayson. Get him before the gets too old, please!)
Michael Angarano (not a great choice, not a bad choice, just a decent choice... nothing more... but he knows a little about martial arts)
David Gallagher (belongs in the category, but I'm not sure he's right for Nightwing. More suited for other heroes, I would say)
Kieran Culkin (good actor that needs to lose some weight before)
Rollo Weeks (could surprise us, but also fail. There's something dark about him that could be to his advantage)
Jamie Bell (most known of the guys. Totally wrong in terms of looks, completely right when it comes to acting talent)
Liam Aiken (youngest, more of a beginner Robin than Nightwing. He's got the looks)
Just suggestions. I don't want all of them, I'm just mentioning possible candidates that too few people here would consider