Since magic is now a planned thing I thought I might suggest this, Over time I've noticed a pattern I find in some fantasy material
Dungeon dimensions (discworld series by Terry Prachett): Home to half existing creatures that feed on and are attracted to magic at points of weak reality, they want to fully exist and be real. Generally monstrous, improbable forms and tentacle-y. They tend to invade the reality through your basic rip in the sky wormhole, by possessing wizards mind or more sneaky means.
(I think Mr Prachett was comically inspired by the Lovecraft books)
Todash Darkness (Stephen Kings Dark tower series etc) Something like the space between worlds inhabited by monstrous creatures. In the last book of
Dark tower series there is a creature from there with too many eyes and tentacles that chased the heros through a tunnel.
"Netherim" world : a sort of different dimension or world in Torchlight 2 game and "The Abyss" : from dead MMO I wont name.
creatures in the "worlds end" level/area from 'Magicka' game
I personally think it should be outworldy insect like stalking, walking, crawling and flying flitting creatures, or some such.
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So basically if there is a mechanic where something like a abuse or concentration of magic leads to merging with or opening to another plane or an invasion of horrifying creatures from another dimension, I think this would add greatly to the "fun" in the game. There obviously isnt enough weird hybrid creatures and hellish attacking demons in the game...