As far as readying an action, at minimum it could work like XCOM's "Overwatch" action, targeting the first enemy you see within range.
But it would be nice to give you a choice of targeting options so that you can designate a smaller area, just in case that's useful. But fine leave it as "first enemy in this area" instead of trying to give you full pencil and paper D&D flexibility. There is a UI for picking between options in an action, such as Enhance Ability needing you to pick an ability.