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I agree with reachzero's post: with CAP1's unique typing giving it resistances to many common Physical attacking types, we need to heavily invest more in Special Defense than Physical so it can stay in on threats like Latios and Chandelure. As for its attacking stats, I don't think they really...
I agree with you that sweeping could be an option on a momentum Pokemon, but most of your examples suck. Scizor's only means of momentum IS sweeping (unless you're running a Baton Pass set, which even then is too easily blocked and set up on) and Crobat can run a Nasty Plot set, but its STAB...
After reviewing the different arguments for each secondary type, I've decided to throw my hat towards the Ghost typing. It gives an all important immunity to Fighting attacks, which will make it invulnerable to them while Roosting (assuming we even give CAP 1 Roost), can operate as a viable...