Yeah, it's most likely DCEU related. However, I want to see Deathstroke return.
I feel like Deathstroke isn't an expendable character even though he may seem redundant due to Deadshot's inclusion. He's just too "important" as a DC character to omit imo. Though I guess the same could be argued for Darkseid or Braniac.
I would actually prefer the entire roster from the first game to return in some fashion, DLC or otherwise. If I had to choose though, Ares, Zod, and Killer Frost would be the least important for a return to me.
Edit: Very rambly, sorry. No caffeine yet and on mobile.