For MK, I've always chosen characters based on how certain normals or specials look and costume.
-MK9: Smoke: the smoke bomb sound effect, look and feel of standing 2 as an anti-air
-MKX: Kung Lao's 4,4 string; Kenshi's base costume, Sonya's 2, 1, U+3
-MK11: Kitana - her base costume, queen...