This is how you would do it:
Tap button: Deals X potency, functions as standard weaponskill.
Press button and hold: Deals X + kt potency, where k is a multiplier and t is the charge time. If the button is released early, the action is released at that potency. When maximum charge is reached, the effect is released automatically and the GCD is activated.
You can take this design approach with both walking and standing casts.
