Almost every member of the Bat Family has had a speaking role so far in the Arkham franchise. Nightwing has not.
So to make up for that, I think they should make Nightwing have his own game.
So if I were in charge of the Arkham franchise, after the 3rd Rocksteady title/ 4th title altogether. Depending on how good Arkham origins is, I would have WB Montreal make a Nightwing game that would be solely focused around how he learned his new responsibilities that come with the independence of working alone.
All and all the games main antagonist would be either
A. Two Face
B. Riddler
C. Deathstroke
The game would begin with Dick Grayson on his final mission as Robin in a one level first mission.
The mission ends with it zooming into his mask and a small narration. It zooms out to show he is no longer in the Robin suit but instead the Nightwing one.
After that, the entire game takes place 1 year later.
Dick is abusing his newfound freedom out of the shadow of the bat. He is making public appearances, fighting in broad daylight, and even almost kills a gangster until Bruce stops him.
When Batman is defeated by (insert antagonist here), and select villains break out from Arkham, Nightwing must use everything the Bat taught him and more to rescue his former boss, and all of Gotham, from a wave of crime overtaking the city.
The game would rely heavily on character development, the Creeper would make a cameo, we would see Tim Drake training to become Robin, and both Jason Todd and ManBat would be boss battles.