The title does not distinguish between the 3 primary types of Metagame Card Design.
Type1) Cards that can be used to make a variety of competitive decks.
Type2) Cards that reduce inefficiencies (OP/UP) in the metagame.
Type3) Cards that add new dimensions to the metagame.
Type 1 card design
1) Make the card flexible with a variety of uses, synergies and combos. Cards that fill multiple roles also work.
2) Make the card balanced with the metagame as the standard.
Type 2 card design
1) Find an inefficiency (OP/UP card) in the metagame.
2) Find a solution that does not cause another inefficiency.
Type 3 card design process is much more difficult. I am but a student here. Hindsight is 20/20. Use it on past type 3 cards like Pendulums, Nova, Fractal and Crusader.