No way. Stuff like Creature Commandoes or Waller are where you’ll see Gunn fully indulge his love for lessers known properties, but a big tentpole movie, one meant to kick off the new DC Cinematic Universe? You do not go with an obscure Z-Lister from Pre-Crisis that only Mark Waid would know. Take a look at how poorly Shazam 2 is projected to open as evidence of what happens when you don’t have an established successful shared universe to get people to go check out a movie full of C-Listers.
For Legacy the two most likely choices are Brainiac or Lobo. Or, as Myskin suggests, one big mastermind villain who is the overall threat (which would be Brainiac, Lex, or Ultra-Humanite) and a bunch of lesser known villains working for him (which is how we could get someone like Bloodsport in the movie). I could see a plot like Arkham Origins where Lex or Intergang or whoever puts a bounty on Superman’s head, and then a whole bunch of villains try to collect. I think that was the basic premise of Up, Up, and Away.