While Elio's not really big on Halloween, he would cosplay for the sake of his friends who are into the holiday - especially if they coordinate a certain theme beforehand.

Off the top of my head, he would definitely dress up as either: Al-Haitham (Genshin Impact), Belial (Granblue Fantasy), Yu Narukami (Persona 4), Zero (GARO), Adam Taurus (RWBY), Taion (Xenoblade 3), or Dan Heng (Honkai Star Rail). Their character designs catch his appeal. It's subjective to change, however, depending on the agreed-upon theme.