I didn't want to say this because it's kinda dark and once the thought's in your head it'll probably bug you forever, but someone else mentioned it on a forum somewhere, so here we go
So we knew Discord would either have to be reformed or destroyed eventually after he was turned to stone the second time because the Mane 6 are mortal, and once they are no longer around to keep him turned to stone, he could get out again with nopony to stop him. His redemption in Season 3, by means of friendship with Fluttershy, seemed a more permanent end to his evil.
But, what happens when Fluttershy is gone? Discord is immortal, so he'd have to spend an eternity without her (as opposed to Twilight, who was confirmed to still be mortal). Would he go Emo and revert to his evil ways? Would he recognize that that wouldn't be what she would have wanted? Or could he teleport to the other side, like how Tirek and the princesses were sent to its lower region Tartarus (hell) while alive, through a portal no less? (After all, Goku can do this in Dragonball Z, even Mario did it once)
I've heard theories that he could make her immortal or bring her back, but either way she'd have to outlive the other Mane 5
Or is it simply another matter of headcanon, since it isn't likely to be explored in the show for how dark the subject of Fluttershy's death would be (beyond comical speculation that she exploded twice)?