Aight, so, not much more left to go here.
Following the defeat of Norishige, the PCs would head to the top of the tower, where they'd find a gateway to another dimension. Ooo. There's also one of the messengers merged with Chikako. And so the big mystery of the missing persons is solved!
...
Chikako wouldn't be doing any talking, so we'd just probably commit to the big battle. And since I don't like to pull my punches with final bosses, it'd be a doosie.
After enough damage was dealt to the monster, it would split from Chikako, allowing you to finish it off with whatever artifact you had. With the keeper gone, the gate would be sealed.
Of course, before you could do that, Sho would show up and freeze y'all, bastard that he is. He'd rant for a while about how only he had the power to save the world and such, and since his power is the ability to do basically anything, you'd be pretty much helpless here.
That's when the one character who's immune to Sho's ability would show up. Jin! Hurray! Jin would totally kick Sho's ass, eventually knocking him out.
But while all that was going on and nobody was paying attention, Ryo would have merged with the Keeper. He'd let you know he was there by knocking Jin out and ranting about all the power he now had. Since I'm a fan of final bosses having more than one form, you get to fight the Keeper again! Only this time he's also got Ryo's abilities, and probably more stats.
And that's pretty much it. After the battle, Ryo and the Keeper are destroyed and the gate seals. Miraculously, the Arcana weren't destroyed in the process. Instead, Winston shows up to explain that the prophecy merely meant that all the Arcana except for Saki lose their powers. Then he'd mention how you were free to live your lives as you wanted. He, for one, was going to finally see Japan. Then he'd leave with Jin.
After that, you'd get to talk to Chikako, then probably some of the other Arcana (Heirophant in particular), and deal with Shun's little death game. Whatever that was. His games all tend to have little tricks, but since you were the ones picking it, I cna't really say what would have happened.
The game would end at Ryuji's house, some years later. I dunno if the PCs would be present for it or if I'd just act it out in channel, buuuut it'd pretty much involve Ryuji, Eriko and Chikako (who winds up Ryuji's girlfriend after finding out Kyon dumped her for her best friend and neither of them told her :P)
They'd reminisce about some of the stuff that happened, mentioning Saki at some point, and then they'd hear a motorcycle outside the gate (Ryuji lives an old style Japanese home, naturally. And then it'd end.
Or alternatively, Saki would just walk in and be like "Hey guys, I'm back." The other one seems more like an anime ending, though, so I was leaning towards that.
Anyhow, that's what happens. Nothing else to say here. All done.
Well,
almost.