Oya (The Magicians)
The "Orisha" or physical form of an African goddess of thunder. Oya was in the top echelons of the West African pantheon of gods and a renowned sorceress. She could hop over to Fillory to teach Quentin and friends a few things about casting spells.
Ryn and her band of mermaids have shown a capacity for compassion and love. Not the Rusalka.
This species of Slavic mermaids are a bunch of total basket cases completely void of the ability to feel remorse. This will show the individuals trying to starve Ryn and her people that things could be worse. So. Much. Worse.
Of all the Jewish Judges of old, none met an end as tragic or brutal as the long-haired strongman, Samson. This swift end came courtesy of Delilah, who if switched for Mazikeen could ignite an exciting look into Mazikeen's past. Because let's face it; Maze's past has been so intertwined with Lucifer's almost to the point of her not having any sense of agency before coming to L.A. The now blind Samson could return to pay Maze back in her own coin.
Huitzilopochtli (American Gods)
Yeah, no one gets the name right on the first try. Like Wednesday, this Aztec war god's powers would have waned significantly, and his nature as a war god means he'll easily buy into Wednesday's call to arms and join his band of merry old gods seeking to reclaim their past glory.
Samael (Legends Of Tomorrow)
If you think demons and evil sorcerers were bad, then you clearly haven't met the being who was the inspiration for the grim reaper. While often confused with Lucifer, this guy's name translates to "Venom of God", and you better believe he more than lives up to it. The Legends have cheated death more than once with their time ship and who better to come in and correct these aberrations than the one in charge of gathering up souls and delivering them to their maker.
Grendel (Midnight Texas)
Another foe of Beowulf's. Said to be a descendant of the biblical Cain; this terrifying creature can return to scare the living daylight out of our heroes or he could join them. We know Midnight is the best place to let your freak flag fly. Plus, Beowulf was kinda arrogant, so it'll be awesome to find out Grendel didn't run off and die in the swamps as he made everyone believe.