Who knows what dumb excuse they'll give for him being around, but one bit of speculation that makes a LOT of sense and is actually interesting is that Steve Trevor is dead.
That's Martian Manhunter.
Who is Martian Manhunter?
Literally a Martian living on Earth. He's a shapeshifter and lives life as a human. However, he's got a power set like Superman plus the shapeshifting and some telepathy and phasing... He sounds uber strong right? He does have a weakness that's really easy to exploit: Fire. So all that power is easily beaten.
He also happens to be a key member of the Justice League.
The best part of him is that WB/DC could cast multiple actors to portray him. First, this would be a very easy way to keep Chris Pine around and not cheapen his death scene, which a lot of female WW fans are already complaining about. Second, you could cast someone like Andre Braugher to portray him when he's not in human form. Braugher would be absolutely perfect and his Captain Holt role on Brooklyn 99 proves it.