Y'know everyone says there'd be no suprise if Superman returns in JL. But there would. If he dies and returns in JL the suprise isn't that he returns. It's when, why and how he returns.
-We know why, to join the JL and protect the world.
-By when I mean when during the movie. Depending on how long after BvS the JL movie takes place we'll have alot of time in between the beggining assembling the JL and Darkseids intial attack for the team to assemble. I assume Superman will return when the JL is being overpowered and give them a winning edge against Darkseid.
-Of course he could also simply land from outer space after a trip off planet to heal near the sun at the very beginning of the movie and the first act of the film could be how people and heroes are reacting to his return. Also how Superman is reacting to the heroes that have risen as protectors in his absence. Wondering whether, at this point, Superman is needed or not.
-Or maybe the first Part doesn't have Superman until the final act. Maybe a different threat rises in light of Darkseid assembling the JL in Part 1, with Superman arriving in the end to even the odds. Seeing this Darkseid decides to pay Earth a visit and the team fully fights together in Part 2.
-Then there's how. Obviously the only thing that can heal Superman is the Sun. So he'll probably go to the sun. Ofcourse, he doesn't have to go directly into space to be healed by the sun. Even without being directly infront of the Sun Supermans healing is way beyond an average humans. Hell he could just stay in the coffin sleep in a "Healing Coma" or some comic bs like that.
That's all I got. Nuthin else really heals Supes other than the Sun and his own alien healing factor.