While Nightwing is generally associated with Dick Grayson, his alter ego’s name was actually inspired by a Kryptonian hero. I could see Krypton having Seg-El don a costume and be the hero Kandor needs. I also have an idea about a Kryptonian Nightwing turning up on Supergirl.
By the end of Krypton’s first episode a lot of pieces are set up for Seg to become a Kandorian dark knight. His parents (and grandfather) were killed in front of him. He has a propensity and aptitude for violence. He has a secret lair to operate from. He has contacts in high and low places. Let’s say Seg realizes that he can’t openly fight the tyrannical government and the Black Zero terrorists as himself but the Nightwing persona allows him to take action outside the system as Seg works his way inside.
One version of the original Nightwing is as Superman’s alter ego when he visited the miniaturized bottle city of Kandor in his Fortress of Solitude. Since Kandor’s environment replicates Krypton he has no superpowers there so Nightwing is sort of a Kryptonian version of Batman complete with a Nightcave, a Nightmobile, utility belts and Jimmy Olsen as his sidekick Flamebird. Since the 1970s was when I was reading Superman comics that’s the version of the Kryptonian Nightwing I’m familiar with though I know there are others.
This idea could be easily adapted for Supergirl. I don’t remember if the bottle city has been directly referenced in the show but let’s assume it’s there in the Fortress of Solitude. Kara , accompanied by Alex, needs to visit for some reason, let’s say a Kandorian has some information she needs. Something bad happens like the person she was going to meet is murdered and Kara is falsely accused of the crime. She has to go on the run and can’t get out of the city until she clears her name. Since she can’t run around openly as Kara-El she assumes a new identity with a mask and becomes Nightwing and her sister becomes Flamebird. Alex is a martial arts expert so she might actually have an advantage over Kara fighting without powers (cue a quick training montage). Nightwing and Flamebird catch the real killer and clear Kara’s name. The sisters leave and have a great story to tell the others.
Will either of things actually happen? Probably not. But I still like the idea.