The DCEU has remained a relatively dark environment over the course of the last three movies, and a Nightwing movie could easily introduce quite a bit of humor and optimism to this landscape.
Right off the bat (sorry, I had to say it), it’s worth mentioning the fact that Dick Grayson is an inherently more lighthearted character than The Dark Knight.
Nightwing lost his parents at an older age than Bruce Wayne did, and those wounds healed differently as he has aged.