I would like to suggest Steel/Flying.
Steel/Flying is pretty well balanced in terms of both offense and defense, which is exactly what we're looking for - Flying is a fantastic offensive STAB to have in this metagame, while Steel in combination with Flying offers a very large variety of useful resistances that support its role as a bulky offensive Pokemon.
At the same time, Steel as a STAB has terrible coverage, which means we can really tailor our movepool to decide just how effective we want this to be offensively - we can easily deny it any Fire, Fighting, or Ground moves if we want it to be walled by certain steel types (unless it forgoes a boosting move in favor of a coverage move, which is the sort of risk/reward trade-off we want it to be making). We could also give this a stat spread similar to Serperior and a more defense/support-oriented moveset if we wanted to.
Steel/Flying is probably the most versatile type suggested so far, only matched by Steel/Water (which is also a great idea for pretty much the same reasons Steel/Flying is). If we want to maintain the goal of our CAP being able to sketch a large variety of moves to pull off a large variety of movesets, this would be the way to go, in my opinion.
Edit: I see reach had the same thoughts as me regarding having just one useful offensive STAB. Grass/Steel would also be good, but I think that leans a little more defensive than I personally prefer.
Steel/Flying is pretty well balanced in terms of both offense and defense, which is exactly what we're looking for - Flying is a fantastic offensive STAB to have in this metagame, while Steel in combination with Flying offers a very large variety of useful resistances that support its role as a bulky offensive Pokemon.
At the same time, Steel as a STAB has terrible coverage, which means we can really tailor our movepool to decide just how effective we want this to be offensively - we can easily deny it any Fire, Fighting, or Ground moves if we want it to be walled by certain steel types (unless it forgoes a boosting move in favor of a coverage move, which is the sort of risk/reward trade-off we want it to be making). We could also give this a stat spread similar to Serperior and a more defense/support-oriented moveset if we wanted to.
Steel/Flying is probably the most versatile type suggested so far, only matched by Steel/Water (which is also a great idea for pretty much the same reasons Steel/Flying is). If we want to maintain the goal of our CAP being able to sketch a large variety of moves to pull off a large variety of movesets, this would be the way to go, in my opinion.
Edit: I see reach had the same thoughts as me regarding having just one useful offensive STAB. Grass/Steel would also be good, but I think that leans a little more defensive than I personally prefer.