Dick Grayson has the unique occupation of being one of the few heroes in the DC Universe that's grown as literally as he has thematically. Fans have seen him become Robin as a teen, leave Batman to become his own man as Nightwing, and later even take on the mantle of the Bat himself. There's no character in the DCU that's travelled the same distance as he has, so any film should, ideally, replicate that element onscreen.
There's no point in taking a character like Dick Grayson and turning him into a proto-Batman, because it'd waste the very evident potential there is in his story. If it's a prequel (and we surely hope that it is), then it would do well to kick off from the moment Dick leaves the cave and carves out his own path in an unfamiliar city, away from Bat-fleck and away from the red and green costume.
Audiences will know who Dick Grayson is - he's the most famous Robin after all - but the whole Nightwing thing is a relatively untested concept for mainstream audiences. It's all well and good to make a 'badass' Nightwing film, but if you ignore the fundamental aspects of the character's journey then you may as well not bother.