While there are many interesting ideas in the previous pieces of discussion, I think we should focus ourselves not only on the "what do we want to have decreased usage"part of the concept, but also on the aspect of "what do we want to have more usage". While "nerfing" a type is arguably not excessively hard (introducing a new strong resistance, or a strong attacker with a SE STAB against that type, you name it - probably there are also other good and relatively easy to implement ideas), changing the metagame to achieve a bigger usage on some lower used types is in my opinion the bigger challenge, depending on what type we choose. For example, some types may be inherently bad, and thus trying to raise the usage of such types is almost an unfesible task if we want the pokemon to have an impact on a strong type at the same time (an example could be Rock; it's hit super effectively by three of the four "strongest" types, while being also murdered by Ground and Grass; I really can't see much room to raise such a type in the current metagame, especially if we don't want to obtain a uber-high BST poke like in CAP4); other ones are heavily hampered to the predominance of another type while also having inherent flaws (Fire, for example: Water-type predominance obviously hurts a lot, but that SR weakness is not helping either). So I think that reflecting on what type could be feasibly "boosted" in the current meta is as important, if not more, than to discuss what we like to gimp a bit.
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