Don't forget the completely new character (100% chance of it being female) with a design that kind of clashes with the artstyle of the whole game, takes the spotlight to themselves and comes with a new ending tied to said character. Obviously the new ending has to be even more awesomer than the ones in the base game, so in the new path you get to unite all the reason bearers by making them realise friendship is the true path to a brighter future, make them join your party and take down Lucifer, Kagutsuchi and, after witnessing your actions and branding you the Ultimate Sinner, YHVH tries to take you down by himself. There's a cutscene before his second form where you're about to be defeated but then all the demons and manikins in Tokyo suddenly start cheering you on, powering up the Demi-Fiend and making him defeat YHVH.
However, that wouldn't be cool enough, it has been done before, so we have to go deeper. After realising this wouldn't be enough to break the endless cycle, Demi-Fiend and his merry troupe of best friends travel to the center of the universe with Stephen's help (he's also a super bonus boss with 3 different phases:wheelchair Stephen, standing up Stephen, and One-Winged Angel Stephen who takes his glasses off and arrongantly smirkes while he flies above the groud enveloped by a holy-dark aura) and meet the Great Will itself. You take down the Great Will, who aknowledges your efforts but says it is ultimately meaningless, as humanity will always desire to be ruled over and yaddaydaddayadda. That's when the new character, let's call her Sue-chan, makes a grand speech about how much she learned from the demi-fiend about the true meaning of happiness, friendship and humanity's value. In an anime cutscene, she reveals her true form: True Goddess Tokyo (Shin Megami Tokyo), and proceeds to sacrifice herself in order to seal the Great Will and finally grant humanity true freedom. Everyone cries and tries to stop her, "Sue-chan!!!" "we can't go on without you!", but she smiles and the screen goes white. She gives a final speech about hope, friendship and freedom, and we cut back to Tokyo (the city).
Demi-Fiend, Isamu, Chiaki and Hikawa are having a friendly meeting. Isamu and Chiaki apologise about what they did back in the vortex world. Hikawa says how you managed to open his eyes and how foolish he had been, and how he is going to work to atone for what he did. They part ways and the player meets Hijiri, who explains how his curse had been lifted thanks to you and drops some obvious hints about how he is, in fact, SMT 2's protagonist, because subtlety is overrated. He reinforces the message of the new ending (in case it wasn't already clear enough) and leaves. You meet with Yuko and you two have a romantic-but-not-really-just-in-case-you-have-another-ship scene, she clarifies some plot details like how Futomimi and Sakahagi came back as humans, worked out their differences and are now best friends, the lady in black's true identity, and briefly reinforces the ending's message again. The screen goes black and a text message appears, heavily implied to be Stephen. He congratulates your achievements, but reminds you that "if humanity falls back on their old ways YHVH shall eventually return" because we can't have the ending be too conclusive, of course. Credits roll, Fin.