3) Deathstroke
Sure, Deathstroke is already set to appear in The Batman, as played by Joe Manganiello, but the character should really make some sort of appearance in the Nightwing movie as well. Either in a major antagonistic role or just a small cameo. The reason is that he’s pretty much Grayson’s nemesis, with Nightwing having a lot more reason to hate him than Batman has.
Deathstroke originated in the comics as the arch-enemy of the Teen Titans and, as such, has brought Grayson a lot of strife over the years. He’s planted a mole within their ranks, killed one of his close friends, taken over the Titans and corrupted the team. He even played a part in destroying Bludhaven, the city Grayson had sworn to protect.
Seeing as he’s in The Batman, we likely won’t see Deathstroke as the main villain of the Nightwing movie – that is, unless he becomes the Loki of the DCEU – but the characters’ shared history means he still has to make some sort of appearance.