I am set dead against Poison-type.
Offensively, it provides absolutely nothing that Dark couldn't do. It's a horrible offensive STAB. Defensively, it grants us one immunity and resistances to only rarely-seen types outside STAB, such as Grass, Poison and Ghost. Although it allows you to take on Psychic, Ghost and Dark attacks you could do that anyway with a pure Dark-type. Thus, Poison-type adds no real resistances defensively. It slightly weakens Fighting attacks at the expense of a Ground-type weakness - not really worth it, in my opinion, especially considering the prevalence of Earthquake in this metagame.
Second point, this could turn out as just another Drapion with a different ability. Drapion has reasonable Speed, a *usable* Attack stat for its STAB Pursuit, and good overall defensive stats. Personally, although others may feel differently, I think it would serve us better in learning about the metagame if we use a typing that doesn't correspond CAP9 to an already-existing Pokemon. The Poison and Toxic Spikes immunity is not really that important, especially if we intend to include an all-around status immunity from our ability, rather like the argument for Dark/Ground at present. It would also make Poison Heal an impossible choice, and since I feel it is one of the top contenders for the ability, I would vote against it, though again this comes back to personal preference again.
I would vote either for Fighting or Ground as the secondary type, simply because not only do they both provide a Stealth Rock resistance, but both give a load of other useful resistances, are both reasonable defensive types, and are relatively unexplored areas; though as Doug has said, Fighting-types have been explored before, so I'm leaning towards Ground at the moment.