The next big breakout character is easily going to be Martian Manhunter now that he's out. I think Reo may have already showcased a bit of why he will be good, but in general he has damage, zoning, pokes, punishes, teleports, and tricky stuff. In short...he can counter any tactic and adapt during any match if you want him too and reap large rewards for it. He just has all the tools.
Following him though is Hawkgirl and Nightwing. Nightwing for the insane setups and pressure. He's really great but sadly his counterpicks are some of the best in the game so folks will HAVE to keep an antizoner backup for him. Hawkgirl only suffers to a couple of folks though and while Green Lantern is probably the best known she suffers against some of her best matchups are the rest of the top tier. She can ruin a Superman player and honestly I dont think Black Adam can run for free on her either. Lots of her flight stuff can get her in on him and once her pressure starts its corner time and 40+ percent damage. All her little punish combos tend to do in the 20's as well and she is HARD to deal with on reaction with her speed and overheads. She might seem to straight forward to breakout but she is hard to react to as Wing Evade gets hard to read and she can cross behind people's wakeup and stuff most wakeup tactics making her very hard to deal with as unlike folks like Bane she doesn't have trouble getting in to start her dirt.
Look for Hawkgirl before Nightwing but expect breakouts from both. Expect MMH before both of those though IMO.