Poison/Dark
Neutral: Normal, Fire, Water, Ice, Bug, Fighting, Rock, Dragon, Steel, Flying, Electric
Resistance: Dark, Ghost, Grass, Poison
Immunities: Psychic
Weakness: Ground
As far as typing goes, this seems solid. An immunity to a move that isnt the most common, and a single weakness to one that is very common, while a couple of neat resistances sprinkled here and there, isn't overpowered and I think that should be what we're aiming for.
Poison gives access to toxic spikes/toxic and possibly haze, while absorbing the foe's T spikes.
Dark gives a lot of neat utility moves... Moonlight, Taunt, Knock off, Memento, Snatch, Thief, Nasty Plot, and perish song to name a few. Moonlight and Perish Song seem to fit flavor wise.
Now, I've talked with Thunderpup, and we've tossed the idea of an Ariados evolution around. Ariados' typing is Bug/Poison, but it isn't far fetched in the least to change it's typing around to fit this. Despite being a pretty bad pokemon, Ariados has access to a lot of utility moves as it is, while having a signature move in Spider Web which functions exactly like Mean Look. In addition to this, Ariados has stuff like Baton Pass, Agility, Sunny Day, and can certainly gain some new moves from it's brand new typing. Spider Web -> Perish Song -> Baton Pass is so much cooler than Mean Look :D
Also, Ariados already has the ability Insomnia, giving it a free switch in to sleepers. An evolution with a Dark typing also gives him a STAB Sucker Punch.
While it's early to discuss whether we want to make this an evolution of a pre-existing pokemon, I think this is definitely something you may want to consider.
Not to mention...
this is an awesome sounding typing.
:]
edit: oh, are evolutions not covered in this sort of CAP project?
Poison/Ground
I was convinced early in this because of it's interesting features; SR immunity, immune to toxic, sandstorm resistant, thunder wave immunity, etc etc. All in all, it's a cool idea, but I like the concept of a Poison/Dark a whole lot more.