Recompletion. I find it hilarious that people nowadays shit on how Roxas and Xion were brought back by replicas, but they are fine with this one. Recompletion is one of the laziest concepts introduced in the series and just a quick and easy way to revive every Organization XIII member after they were killed off. Not only did they make the Organization unkillable thanks to this, but the concept of becoming a Nobody has now lost all meaning because now all you have to do is kill yourself and poof you're back to normal. No need to build a "fake" Kingdom Hearts moon. Yen Sid tells Sora to destroy Xemnas but fails to alert him that doing so will bring back the even bigger threat of old man Xehanort. Recompletion either needs to be a super rare thing that is complicated to pull off or retcon it out of existence.
Roxas's two Keyblades. So anyone who plays the game will see after the Xion fight Roxas gains two Kingdom Keys that become Oathkeeper and Oblivion. Naturally you'd think 'Oh, he has Xion's power to wield her replica Keyblade. That's where the extra one from KH2 came from.' This is one of the few things the games, manga, and novel all kind imply on something that's not 100% clear. But nope, Nomura said that damn Keyblade is Ventus's. Never you mind it was a Kingdom key before transforming. Why did they insert Ventus into something that had nothing to do with him and works perfectly fine without his inclusion? This just opens up a whole can of worms of the whole 'Ventus was in Sora's heart since the start of the series' just making the Sora/Roxas connection more complicated then it needed to be. I'd retcon this!
Sheesh, it feels like all my posts now are just mega rants. Have I really become the grumpy old Kingdom Hearts fan who yells at heart-shaped moons? Regardless I'll probably have more coming.