Re: Stuff you like to believe happened in Nolan's series
I agree, I wouldn't want to actually see a movie with him as Nightwing because then it does rob Dick Grayson of that spot in a future movie. But as far as imagination goes, I don't have any problem with thinking of Blake as Nightwing, because in the Nolan-verse he effectively is Grayson, Drake and Todd all combined, a new take on the Robin character. Since they adapted elements of Grayson into that character, I figure the Nightwing persona could be just another element.
It's no less valid to imagine him as a Terry-esque Batman though, or some kind of variation of the Batman persona. My reasons for leaning Nightwing are mostly superficial (IE, I think JGL really looks the part, the chalk symbol resembles the Nightwing logo, etc.)