This is how it should be.
GK: one-on-one stopper, penalty kick stopper
DC: aerial defender, defensive wall
DL/DR: aerial defender, defensive wall
DMC: aerial defender, defensive wall, playmaker, corner specialist, free-kick specialist
DML/DMR: aerial defender, defensive wall, dribbler
MC: playmaker, corner specialist, free kick specialist
ML/MR: dribbler, corner specialist, free kick specialist
AMC: playmaker, corner specialist, free kick specialist, shadow striker, dribbler
AML/AMR: dribbler, corner specialist, free kick specialist, shadow striker
ST: one-on-scorer, dribbler, free kick specialist
Any player can be a penalty kick taker if he has the qualities for it.
Other special abilities that could be added are: hard tackler (defenders and defensive midfielders), one-on-one defending (central defenders), leader (captain), and poacher (striker).