I think Homeslice and Jynx KB have a good point in mentioning (offensive) cores. (And giving Offensive Ice as a decent example of what would create a good core)
As cores always revolve around more than just one pokemon, most often two and sometimes even 3, if we can invent a core that utilized less common typings, but together can work to combat the more prevalent strategies like Rain or Dragon spammage, you've effectively fulfilled the concept.
Even more important, when you know you can rely on the power of other pokemon to combat Dragons, Waters, Steels and Fighting, and are actually building around the qualities of other lesser used pokemon, CAP5 itself doesn't have to be ludicrous in it's capabilities. The only way to make CAP5 counter all the common mons that sport those typings is to something close to Arceus in stats and capabilities. That is, if he'd have to perform the job all on his own.
I think we could definitely create a mon that is somewhat reliant on lesser used helpers, but when used together, they can quite effectively combat some of the more commonly used types/strategies.