Ok hear me out
What if the reason why the DK and SD were so angry was that Elizabeth was pregnant. Meliodas knowing of the upcoming battle asked OG Gowther for help. Gowther knowing that after the Gods dealt with Meliodas and Elizabeth, would fight over possession of the child, used his magic to erase everyone’s memory of the child ever existing and hid the child. Instead the Gods just cursed both for their relationship and Gowther ended the war by changing Mael to Estarossa.
Now the child of prophecy let’s just name him Tristan was isolated from others and due to him inheriting darkness from Meliodas and the Light from Elizabeth had severe bipolar tendencies. In places of harmony, he would lose control to the darkness and bring destruction. In places of war, he would be guided by light and bring peace. He had no control over these tendencies and was afraid of himself. Over time, he harnesses his power and awakens his inherent magic, “balance”. That is if the darkness or light ever increase, the other will compensate restoring balance. For example, if he absorbed the commandments he wouldn’t lose control because the light would increase and he would become more powerful. With his magic, he can restore balance to places that have been affected by natural disasters. Also, he could remove curses or spells like the ones Merlin cast to keep her youth. So, upon achieving this state, he feels the obligation of overseeing this world and maintaining it. Also, he is able to fuse ark and darkness to make fusion attacks which are lethal to non goddesses and demons. He will have an appearance with large goddess wings covered in black, yin yang eyes and the demon mark with the ark color.
Now his appearance will be as the final antagonist. Imagine Meliodas and Demon King fighting at their peak is causing the world to go out of balance. The Demon King is about deal the final blow, but then he appears in flash piercing right through the demon king with ark. The sins shocked at the new warriors sudden appearance watch as he uses his magic to remove the commandements from Zel and absorbs them himself. Demon King makes an epic face as he disappears and Zel lifeless corpse is thrown to the ground. As he’s covered in darkness he faces the sin, and then you see his eyes return to the yin yang state. He then declares he will eliminate Meliodas since his presence is not required and doesn’t want his interference in future matters. Since he watched from afar, he blitzes Merlin smacks her with fusion attack and uses his magic to remove the spell she cast on herself, and !merlin turns to dust. Escanor in anger turns into the one and retaliates, but is no match, and Tristan uses a fusion spear to incapacitate Escanor, and removes the sun and absorbs it himself. Also, during this time Meliodas is heavily wounded and being healed by Elizabeth and the sins stepped up to fight. Next, Diane goes to attack and Tristan moves to her blind spot but anticipates Kings response and in turn rips off his wings and throws him full speed at Diane’s head. Gowther shoots an arrow because of balance no effect, and Tristan severs Gowther head. Ban goes for Full react but is too slow and then tries Snatch but because of Balance no effect. Tristan counters and notices Bans body is tougher, so Tristan uses a fusion attack and knocks Ban into a coma like state.
Now Meliodas is fully healed and they spar, but is no match. Meliodas goes to butterfly form but still not a match. As Tristan goes for the kill, Gowther restores everyone’s memories of Tristan. Tristan’s background story is seen and momentarily the darkness overpowers him. Elizabeth goes to comfort him and his eyes return back to yin yang and he goes to touch her face, but then smirks and hits her with darkness. Now about to kill Meliodas, King Arthur appears and saves them. They fight and Arthur is not overwhelmed, but Tristan is testing him since he doesn’t see him as an enemy and then tries to talk Arthur to his side. Arthur seems moved by his words about Meliodas being unreliable and the sins too for supporting him. They both could push the world in a new direction. But, upon learning of Merlin’s death, Arthur changes his mind and attacks. They’re evenly matched, but Tristan quickly overpowers him and knocked him unconscious. He takes Excalibur for himself and moves to kill Meliodas. Then, the world tremors loudly and the demon king and SD appear, with the Demon King swinging at Tristan. The DK and SD both fight against Tristan. Tristan is able to fend off both of them, but it unable to gain an advantage due to both of the fighting in sync. SD is almost full power since she has the three graces returned to her. The DK and SD combine for fusion attack and Tristan counter. They are enveloped in a beam struggle. While this happening, the SD restores life to Zel, Ban, King, and Diane. Though they’re still heavily injured. Ban uses snatch to take power from everyone and than gives it to Meliodas, who will then use his original magic to erase Tristan. Meliodas does in fact succeed, however Tristan is able to make contact with Meliodas and share the effect. Elizabeth seeing this jumps in, and all three are erased. The DK and SD end their long standing battle, Zel and Mael lead the clans, and are separated from the mortal realm.