So since the internet is currently lack a P5 easter egg list, I decide to open this discussion to gather them in one place so it'd be easier to see.
The first I notice is a Homestuck reference as this tumblr post state: spoopy-ceetee.tumblr.com/post/159859454196/glad-to-see-that-atlus-is-still-homestuck-trash
Spoiler: it appears during the 1st celebration.
Too bad I missed it.
There's also Naoto Shirogane's name being mentioned early on the TV, Rise Kujikawa's poster, and a equipment that referencing Sonic (I forgot what it is, but it's a shoe that increase agility and said it "makes you run as fast as a hedgehog" or something like that).