Adding a hold or double tap would be quite cumbersome and very hard to use effectively, since a lot of the times its used impulsively. Also, most of the other melee commands in other games are just the single tap, so by default you'd only press once quickly and expect your melee to do its thing. However, not being able to choose when to use the powers is a problem, maybe add a toggle in settings to 'always use when available' or have an option where u can hold another button at the same time for the power.