Nyssa Raatko - I could see her coming back in one of two ways. 1) She could target Damian Wayne as a way of hurting her sister and father or 2) She could reform Leviathan and this time be a villain squared off against Nightwing who would need to pull in his Spyral contacts to battle her.
Double Dare - could fill in a niche in Nightwing as international thieves that he has to track down and capture. Instead of them being small time crooks and acrobats in a circus have them perhaps be a feature act and part owners of a major circus and be big time international paid thieves.
Fright - Have her working as a lieutenant to Blockbuster, perhaps hired by him to try to reverse where Nightwing "cured" him. She could be both Blockbuster's personal physician and someone he turns to interrogate someone.
Jane Doe - with her talents the possibilities are endless, especially where she tends to lose herself in the person she replaces.
Lady Vic - She was the most interesting of the villains Dixon created for Nightwing during his run. Perhaps pair her up with Deathstroke.
Magpie - She showed up briefly in Batgirl, but there is loads of potential here for a good villain.
Mali - She was interesting character from late in Higgins Nightwing run. She is more of a "grey" character which makes her more sympathetic to the reader.
Scarlet - could be fun to have her return and either target Robin or Red Hood.
Sister Zero - She could be an interesting villain to have show up, especially with the engagement of Batman and Catwoman.
Tarantula - This character began with such promise until she along with D Grayson's entire Nightwing run went off the rails.