Fire/Poison
Ice/Rock as most people said leaves no room for creativity. With that Dual STAB, it's either going to be an offensive Powerhouse (which we already have in CAP) or a huge failure that gets rocked by every single priority move in existence.
Fire/Poison is one of those types that looks bad on paper, but as many people pointed out, it resists three forms of priority and is immune to two common status effects. Since it is a Fire type, we can be flexible about it being a physically offensive threat or we can attempt to use its unique typing as part of a defensive core (I'm thinking with something like Celebi it could be cool)
All in all, Fire/Poison gives us the most flexibility for this CAP project.