Pretty self explanatory. Please try to keep them balanced, and include reasons if you want.

Arcane type.
Basically a "magic" type. Psychic and fairy are similar enough, but this is based more on "classical" magic, with rituals, potions, and shit.

Strong against: Dark, Ghost, Fairy. (magic can be used against supernatural creatures)
Weak to: Grass, Steel, Ice, Fire. (materials and temperature are very important in rituals and potions)
Resistant to Arcane, Dark, Ghost, Fairy, Poison (same reasons as "strong against", plus self resistance and poison resist because of potion knowledge)
Resisted by Grass, Steel, Bug. (Bug resists is there to buff it a little, and some bulshit about nature resisting magic)

Gameplay wise, Arcane type would focus mostly on Special Attack and status moves.

Some pokemon that would be retyped to Arcane are Delphox (Fire/Arcane), Abra line (Psychic/Arcane), Mismagius and Spiritomb (Ghost/Arcane), and Sigilyph (Arcane/Flying).