I'm not sold as to why we need another competitive ability. Weak Armor is a fine risk/reward ability in that it promotes switching into Physical attacks, which means that in order for this ability to activate, you're going to need to take some damage. Great choice.
However, since Weak Armor is such a great ability, why do we need another? Weak Armor accomplishes the concept very well, and adding another Ability seems to be adding to the Pokemon simply for the sake of adding to the Pokemon. There are plenty of good suggestions here, I particularly like Analytic and Rivalry. Though bmb has made his stance on NCA pretty clear, I still think it's the best choice for the CAP.
Complaining aside, Analytic and Rivalry are my two favorites for the secondary ability. Analytic rewards lowering this Pokemon's Speed, which forms a very interesting duality between this Pokemon's two abilities. Meanwhile, Rivalry will make players even more conscientious about gender selection, which will add interesting mindgames to teambuilding when it comes to gender selection. Rivalry might be a bit unsophisticated, but it does accomplish the task.
Analytic, however, is a very interesting ability in the context of Weak Armor. Analytic will (THEORYMON ALERT) split this Pokemon into two very different sets. Weak Armor will most likely promote running Speed on this Pokemon to fully take advantage of the ability, while Analytic will promote running bulk and low Speed in order to take advantage of the power boost and slamming a switch-in immediately. Analytic is risky in that if this Pokemon is given enough Speed to abuse Weak Armor, it also has to have a low enough Speed to abuse Analytic on a different set. The rewards to both abilities are pretty apparent.
Analytic is the best choice for this Pokemon, with NCA and Rivalry as good alternatives. No Guard is a bad choice (high reward, low risk), and many other suggested abilities (Justified, Defiant) are either luck-related or just too small-scale to be useful.