Crack-Theory-Time;
Discord. Imprisoned for a thousand years or two, the first thing he does upon escaping his eternal prison is launch into what is clearly a pre-meditated plan. He inflicts chaos of unprecedented levels on Equestria and outright Dares the wielders of the Elements of Harmony to try to imprison him again. After defeating his distractions and getting everything together, they fire the elements at him, and he is reduced to a statue once more.
All according to Keikaku. (Note - Keikaku means plan).
Discord's real goal here isn't revenge, it's freedom and safety. He knows that Celestia dare not leave him free, and that she will send the Elements and their wielders after him, so the only way to be free of his statue prison
is to convince them that they have won.
He overplays the chaos side of things, turns it way up to 11 and makes himself out to be the biggest, most insane and uncontrollable godlike madman and goads the Wielders of the Elements of Harmony into giving him everything they've got.
When they finally beat his apparent plot and activate the elements, he plays along, substituting an illusion of himself for the target and making a real statue out of it as the elements hit empty space. One lightshow later, the ponies who were too angry to be thinking clearly and too relieved to have clearly won, (as chaos subsides), are convinced a dangerous lunatic is sealed away forever, once more. Discord is free to drop the malicious trickster act (which he cribbed from his own, pre-imprisonment self and turned up to 11), and leaves equestria to take up a quiet life of only occaisional metaphysical sillyness in a land outside of Pony control. He walks away in disguise, to slightly familiar
sad walking away music. (But finally free).