Believe it or not, they managed to make Doctor Snapped even more horrifying.
The extreme closeups of the Captain's ghost in Doubtback.
RIGOR. MORTEX. Quite possibly the most hellish designed villain in the entire series. Especially when it uses a spell - its eye focuses in its center, spins around, and then SUDDENLY AND TERRIFYINGLY BECOMES BLOODSHOT AND SLAMS ITSELF ONTO THE GLASS, FILLING THE ENTIRE AREA. Jump Scare to the extreme.
The fact that completing the Monster Library requires multiple copies of Captain Crow, with the Final Boss alone requiring two of him, as unlike in the original Joker he's Lost Forever upon synthesizing him away. Getting more Captain Crows requires tagging them off another copy of the game, which is significantly easier to do in Japan than in the West. Professional altered several synth chains so that only one monster (Xiphos the Deathbringer) requires Captain Crow, but alas...
...and with that said, Professional introduced a new one in that several of the highest-end monsters, such as Corvus and the new True Final Boss, require the Wi-Fi prize monsters in their synth chains. Again, getting these is significantly harder in anywhere other than Japan, and that was before the termination of the vanilla DS's Wi-Fi in 2014 rendered them literally impossible to get without cheat codes.