As a designer I think you can make cards with the idea of the metagame, You can have 3 aims :
1) the cards create a new deck in the metagame : it is a very strong card that can alone be the center of a deck (ex : graboid, GotP...)
2) the card fit in a weak deck, to make it better
3) the card counter a good deck in the metagame, to make it more varie (example : sanctuary which counter discord/BH deck).
or maybe 4) the cards have no impact in the metagame (ex : graviton guard)