I already have a few types ready to go:
Devil-type: Meant to make the Normal-type offensively and defensively valuable. The Devil-type is super-effective on Magic, Poison, Fighting, Psychic, Ghost, and Dragon, and is not very effective against Normal, Steel, Dark, and Mystic. The Devil type resists Magic, Poison, Ghost, Psychic, Dragon, and Dark, and is weak to Normal and Mystic. The Devil-type represents pure evil (as opposed to the Dark-type, which is sometimes sinister, but
for the most part just misunderstood.) The Devil-type is strong against Magic, Poison, Fighting, Psychic, Ghost, and Dragon because these types tend to be vulnerable against temptation. The Devil-type resists all of those except Fighting for the same reason (Fighters are noble in spirit, and while a Devil can lead them astray against their better judgment, it's still gonna be in for a butt-kicking if its deception is found out.) Devil-types also resist Dark-type moves because they're no strangers to dirty tricks; they practically
invented the art of deception, and so are very difficult to get the upper hand on through trickery. Devil-type moves are resisted by Steel (which just can't care one way or the other about this whole good vs. evil stuff,) Normal (which is too pure and innocent to fall for evil's schemes), Mystic (which, as the embodiment of good, will adamantly refuse to let evil get a word in edgewise), and Dark (which is more than street-smart enough to realize that following the advice of a Devil can only lead to trouble.) Devil is weak to Normal and Mystic for the same reason that those two types resist it; their sheer innocence/virtue overwhelms the Devil's evil power and turns it aside, leaving the Devil wide-open for a righteous ass-kicking. Devil-types, as the avatars of evil, tend to have very good stats, and so non-Normal (or Mystic) types may have some trouble with defeating them. (Devil-type moves
will activate the Justified and Rattled abilities.)