Litten definitely has the leviathan cross/brimstone symbol on it's head (eyes + markings just above them). That + the dex entry part that mentions "when it sheds, its fur catches on fire" is giving me hell-ish vibes. I'm guessing it'll be engulfed in flames for it's final evo. (Black tiger with red stripes, on fire, perhaps?)
Hell-based fiery tiger? Now I've definitely got a feeling it's going to be Fire/Dark.