Steven Yeun was recently written out of The Walking Dead with the Season 7 premiere that saw Glenn and Abraham killed off at the hands of Jeffrey Dean Morgan's (Thomas Wayne!) Negan, and checking the IMDb account of Steven Yeun shows that his schedule looks to be pretty much open.
Back in September actually saw Twitter light up with the possibility that Steven Yeun could be playing Nightwing, and there have been rumors of Nightwing, which actually include the possibility of an HBO show. Nightwing was also said to get a reference in Batman Vs. Superman, which didn't happen, but could have been cut.
Obviously if DC goes with Steven Yeun as Nightwing it would be a different direction than the comics as Steven Yeun is of South Korean descent. Is that a big deal? It probably wouldn't change much would be my guess.
In the comics, the Nightwing Dick Grayson is originally Robin to Batman, but later goes out on his own and adopts the Nightwing persona. When Batman first meets the young Dick Grayson, Dick is an acrobat who watches his parents get killed (similar to Batman), and Batman eventually takes Dick Grayson in. Not only is Dick Grayson Robin and Nightwing, but Dick is also the leader of the Teen Titans.
Interestingly enough, at one time WB and TNT were planning a Titans series featuring Dick Grayson, but its since been apparently cancelled. Perhaps the reason being because they want Nightwing in the DCEU? In the planned Titans TV series, Dick Grayson would eventually become Nightwing and worked in Boston as a private detective.
We do see that the Ben Affleck Batman in the DCEU is older, so it would be safe to assume that Nightwing already exists. The Jason Todd Robin suit is in Batman Vs. Superman, so Batman definitely had a sidekick at one time or another.
Would Steven Yeun make a good Nightwing? Check out some awesome fan art of Steven Yeun as Nightwing which may make the decision a lot easier (from Spidermonkey 23).