Gonna go out on a limb here for the last three DLC characters
-Atom
-Booster Gold
-Mera
There might be some bias here.
Atom seems to be a given based on the teeny tiny silhouette. Female Silhouette could very well be Mera and the shapes around the head area could very well be Mera's headdress. Relevancy clearly has little or nothing to do with who NR picks, but Mera is a FAR more prominent character in comics than Enchantress ever was.
Based on the character select scree, I'm assuming Hellboy is the top, leftmost slot. That leaves the bottom rightmost slot which seems to be throwing everybody for a loop. Just my take, but I see that outline and I see a high collar. Only DC characters I can think of with high collars are The Question, Rorschach, Constantine and Booster Gold. Of those four, Booster would be best for a fighting game imo. The first two bring nothing to the table. Constantine would be weird since from what I've read with him he's a trickster, strictly non-combat but they could make it work.