This is a personal wish I'm sure will not to come to fruition but hey, one can dream.
Assumptions
-a Mother Box will be located in Atlantis
-Mother Boxes are capable of restoring life i.e. BvS Cyborg scene
-DC and Warner Bros have trouble integrating the Justice League Dark into the DC Extended Universe with a stand alone film
-DC and Warner Bros need more powerful heros in the DC Extended Universe to fight Darkseid
-Swamp Thing has enough name recognition from earlier live action incarnations
Aquaman End Credits Scene
The Mother box held by Atlanteans has to be moved and hidden because it's location is now known. Wayne Enterprises funds a bio-fuel research facility deep in a Louisiana swamp. The facility is headed by botanist Dr Alec Holland. Batman believes this to be a safe, secure, and unknown location to hide the Mother Box.
Swamp Thing Movie Plot Synopsis
Evil Dr Anton Arcane scientists/dark magic practitioner either by his own ambitions or under order of Darkseid ,Lex Luther, or both learns of the Atlantean Mother Box's whereabouts. Arcane and his monstrous henchmen attack the bio-fuel facility. During the raid an explosion kills both Holland and Arcane. Hollands body ends up in the swamp where the mother box then regenerates Holland's body merging him with the swamps plant matter, the elemental Swamp Thing is born. Through his dark magic practices Arcane resurrects himself as an undead monster and returns to resume his pursuit of the Mother Box.
Mean while a demon hunting exorcist by the name of John Constantine learns that a soul with plans of great evil has escaped hell. He heads to the Louisiana swamp to dispatch it. Constantine first encounters Swamp Thing, believing him to be the unholy hell beast he seeks.Constantine realizes that Swamp Thing is not a monster but something else and Arcane is their common enemy. The two team up to stop him. Hunting down Arcane and his minions. Thwarting his plans of retrieving the Mother Box and sending him back to hell. Arcane vows to return again.
Swamp Thing End Credit Scene
Batman discusses protecting earth from dark supernatural threats with Swamp Thing and Constantine. Constantine says, he might know some people or things that can help. Which leads to the formation of the Justice League Dark.