Name: A Weight On Your Mind
Description: A Pokemon which specialises against heavy opponents.
Justification: Some of the best Pokemon in the game are incredibly heavy - Ferrothorn, Snorlax, Hippowdon and the pseudo-legendary Pokemon, to name a few. What happens when a niche Pokemon capable of punishing these Pokemon for their weight exists?
Questions to be answered:
* What happens when common Pokemon come under threat due to their physical size?
* Is there enough variety in the 5th Generation for such a niche Pokemon to exist and work?
* How much do light Pokemon benefit from a new Pokemon which is ineffective against them?
Explanation: There are enough heavy Pokemon for this design to be open-ended to allow for almost any role, but regardless, it'd lead to us seeing interesting concepts form surrounding what happens when the heavyweights have to watch out for a Pokemon designed to punish them. There are few weight-based moves in the game, but one is ubiquitous (Grass Knot) and the other would see more use if it wasn't for Close Combat's existence (Low Kick).
Given the two types available, it seems likely that this design would have to be either Grass or Fighting-type, but that's not necessarily the case. Many Pokemon have access to these moves without sharing a type, and the lack of a STAB bonus could be compensated through its Ability or simply having high Attack stats.
The presnece of a niche counter like this could easily dissuade players from picking heavy opponents, which could have dramatic effects - both Sand Stream Pokemon are very heavy, so this CAP's presence could be a major blow to Sandstorm's dominance; at the same time, Ferrothorn, Gyarados and other central Rain team members (along with Snorlax on Sun teams) may also be vulnerable. It would likely make the metagame less centralized around the fatties of Pokemon.
It also has several counters basically built into itself in the form of good, lightweight Pokemon, such as the 600 BST pixies (as an example). At the same time, these aren't set in stone as counters, merely likely candidates to a weight-based CAP.
What's also interesting is seeing what this CAP would be capable of in Ubers, with both Groudon (the heaviest Pokemon ever) and Kyogre (also extremely heavy and with a weakness to Grass Knot) present, along with other unusually heavy enemies. That's more of an aside to the main focus, however.
What's the difference between mainstream religion and Scientology?
A few thousand years.