@Null would be the final boss of the story mode and banned from multiplayer due to his overpowered combos and complete lack of fucks to give.
No, Null would be the the final opponent of the tournament, but before the fight can begin a sequence that plays out like the a combination of the beginning of Subspace Emissary & World of Light happens which results in everyone getting killed except Null. He sets out not just to revive everyone from Kiwi Farms, but also save the world which involves fighting many powerful foes, but meeting allies just as powerful, of course allows for many guest characters. For example one of the side stories would be Chin (Hong Kong 97) & Trump (Barron, who's thrust into the presidency after the death of his father) bonding over immigration.
While there would be many villains I don't think any of them would be connected to the main antagonist, for the most part they'd mostly be opportunistic assholes wanting to takeover the world while it's in chaos. For example FuckYou would appear as a reoccurring gag boss who wants to kill everyone, but only has a weak punch as a attack, and has 1 HP, he ends up dying to Couch, who's the final player character unlocked, who'd basically be the version of Null you're describing.
The true final boss would be a corrupted Christine Weston Chandler, who has been taken over by god, having grown sick and tired of humanity has decided to destroy every universe except CWCville, which after everything else is destroyed they will retreat to so both he and Christine will live forever, and they nearly succeed. However god can only control those who are willing to be control, the reason he took over Christine was he figured that if he saved CWCville she would do anything he wants, also god has very odd tastes in woman. However in a change of heart realizing the horrors she and god are committing Christine does the ultimate sacrifice. She turns against him and traps herself and god in CWCville, she then destroy's the universe they're in, successfully destroying god, with Christine and the citizens of CWCville dying happy, knowing what there purpose in live was. Thus the main story ends with Null in shock and awe vowing to make sure that everyone knows the heroic deeds of Christine Weston Chandler.
However the destruction of god cause the worlds religions to go ape shit, but a revived President Donald Trump and his new vice President Barron Trump are ready to take on this threat, the post-game content begins!
I spent way too much time on this.
