@Bloody:
Unfortunately its probably too late for her to have any relevance in the story
She was equal to the DK in strength, and seeing how hes presumably been vanquished for good, I don't see how she can pose any real threat to the 7DS currently unless Nakaba retroactively makes her much more powerful than him.
Sure she can pose a thread… Now back when Ban listened to the horn of cernnunos, it was clearly hinted that she would be battling meliodas. The original premisa was really good at that moment: two lowers destroying certain status quo are hated by goddesess and demons... Next we saw the SD and the DK cursing, together, the lovers. If now the SD appears, well that will be better than nothing but, yes, Nakaba would have chosen to split the enemy force into two halfs... and yes, the best final battle would have been the DK and the SG together vs the SDS. To me is like he was too shy to handle the entire premise of the story. And at ceratin point his reluctance to make the SD appear in the story only weakened the story more and more.
Well maybe the SD revive the DK to make the perfect enemy force again, but if watching the DK figthing the protagonists for a second time was a bit too much, a third time would seem be very dissapointing.