Actor Allusion: Peko is voiced by Kotono Mitsuishi, AKA Sailor Moon. When she masquerades as Kira Kira-chan, she wears a Magical Girl mask that greatly resembles Sailor Moon and makes a speech about punishing people with justice. Since Mitsuishi was one of the voice actors that the script writer specifically asked for, this was most likely done intentionally.
In the English dub, Hinata is voiced by Johnny Yong Bosch, who voiced Yu Narukami in Persona 4; fitting, since both games revolve around finding the truth. However, he also voiced Tohru Adachi, the seemingly-harmless cop who is driven by his envy and misanthropy to try destroying the world. Sounds an awful lot like pre-amnesia Hinata and Kamukura Izuru, doesn't it?
Acting for Two: Komaeda's voice actors in both the Japanese and English versions also voiced Naegi from the previous game. It falls into this trope when Naegi shows up. There's also Akira Ishida and Jason Wishnov, who both voiced Togami in the previous game and again here. It falls into this trope considering that the Togami we meet here is an imposter, and the real one shows up later.
Fan Nickname: Togami as he appears in this game is known by fans as "Fat Togami" or "Fatgami", due to how...different he looks from when we last saw him. "Twogami" and "2gami" are also popular, given how he's someone completely different from the actual Togami. In Japan, "Tongami" is a very popular nickname for him.
Monobear can be considered one too since his official name is still Monokuma.
Komaeda originally had glasses, and a hoodie with bat ears that eventually became Nanami's.
Nanami greatly resembled Chihiro, the person who made her.
Kuzuryuu's prototype, of a big, muscular man, eventually became Nidai's design proper. Nidai's original was much more standard, with no crazy muscles.
Hanamura was a standard tall and slender Bishounen.
Mikan had shorter hair and more of a Mad Scientist vibe, being covered in stitches (in fact, one sketch has her hand falling off at the wrist) while carrying around a Brain in a Jar.
One of Nidai's early designs was a Long-Haired Pretty Boy. Another was done in the more cartoonish style that was ultimately used for Hanamura, depicting him as a rail-thin teenager with a shaved bald head, oval eyes and buck teeth.
Saionji was one of the survivors while Kuzuryuu was the victim of the third chapter in the beta version of the game.