[@Jaredthefox92]
I know those two characters enough to work with them. This is taking part in Modern Age comics, but aside from that you can take inspiration from pre and post New 52, Rebirth or any of the film/game universes (maybe not Injustice). If you want to use Giganta from the Bronze or Silver age of comics, it doesn't impeach on my plans. The same goes for Killer Frost.
Dark and Gritty characters like Batman and Joker exist alongside more happy characters like Plastic Man or Shazam.

[@DocRock]
Deathstroke is someone I would like to see. (If no-one plays as him, he's definitely an npc)
Killer Croc is another character who could work.  Scarecrow, as you mentioned earlier, could be cool.
Honestly, just look up some characters on dc.wiki and find someone you like the sound of. There's very very few that I will say no to, even if you find someone incredibly obscure dude with powers that are stupid as hell.
For example, a few days ago I was reading a collection of Bronze Age Batman Team-up books and it had a villain called "The Moulder" who was just a guy in a hazmat helmet with spiderman web shooters but with hot plastic instead.