Of the types on which people have focused so far, I favor Fighting for its Stealth Rock resistance, excellent STAB, and cancellation of meaningful weaknesses with the primary type. I would be reasonably happy with this result.
However, I would like to recommend that we take a second look at Dragon. Many people seem to think that choosing this type automatically takes the Pokemon down a glass-cannon, offensively-oriented road. Also, I understand the objection to another Dragon-type right after Cyclohm. However, Dragon has several strengths which I think require consideration.
1.) Strong defensive typing. Steel has been largely written-off because it would give CAP9 a 4x weakness to Fighting. I agree with this reasoning. However, Steel was first suggested because it is such an overall strong type defensively. Dragon is similar in this regard, without the crippling Fighting weakness. A Dark/Dragon CAP9 would have the following:
Resistance: Dark, Electric, Fire, Ghost, Grass, Water, Psychic (immune)
Weakness: Bug, Dragon, Fighting, Ice
This is not exactly Empoleon, but it is an excellent selection, one which would allow CAP9 to repeatedly switch in to opposing attacks from numerous threats. This creates compelling synergy with one of the project's other primary goals: minimizing the impact and relevance of entry hazards. And while it is not itself resistant to Stealth Rock, I believe that this has been overemphasized thus far, since there is a lot more to "the secondary" than just that. Adding an ability like Levitate, Magic Guard, or even Soundproof to a Dark/Dragon could still give phaze-reliant stall teams absolute fits.
2.) Strong offensive typing. This is not as good a case as the above, but Dark/Dragon is still excellent from an offensive standpoint, even if it's no Kingdra or Revenankh. Dark and Dragon are collectively resisted only by Steel, and Dragon adds a useful super effective STAB against such persistent threats as Salamence and Flygon, giving CAP9 added relevance against teams other than the obvious Stall.
3.) Flavor. A Dark/Dragon CAP9 would allow us to explore two rich, hitherto underexplored typings in terms of flavor concepts. As several users have pointed out, Dragon has always meant one or both offensive stats reaching at least base 100 and a diverse movepool of non-STAB attacks including potent options like Earthquake, Fire Blast, and Surf. Dark on the other hand, has remained nebulous, used mostly on dual-type Pokemon with no really consistent thread between them. Murkrow and Darkai suggest that it may be tied to the night, so what about a "sleeping dragon" Pokemon? This plays to a stat spread less focused on hyper offense (which everyone seems to agree CAP9 should avoid), and would invite an interesting new take on both of these types. By contrast, Ground and Fighting are both well-explored.
To me, the best argument against Dragon is simply the risk of similarity to Cyclohm. It would be another secondary Dragon, and Cyclohm did already have Shield Dust, overlapping slightly with this concept. This seems like a really inadequate criticism, as the goal of CAP9 is to produce an interesting and relevant new contribution to the metagame. CAP has reused types before, such as between Revenankh and Arghonaut, so I would not hesitate to do it again if it is the best choice for CAP9. I believe that, in this case, it is.
Adding Dragon would make it vulnerable to Latias who is a strong Trick user. I think it would be counter intuitive and it provides very little help with secondary moves. Defensive typing is nice but Steel was suggested for it's Toxic immunity, resistance to Stealth Rock and Sandstorm. Not solely for being a good defensive type.
Dragon, to me, is redundant in style with Dark, both geared towards offense in this case.