So, you'd hold the 'Play' button for a split second, and this menu would pop up (probably at your mouse cursor's position), then you just move your mouse cursor in a direction to choose the target of the card, and let go of Play to actually 'play' it. After having it in the game for a while, I think players would get very proficient at using it, allowing them to play cards at much faster speeds than we currently can (thanks to having to tab back to the boss each GCD to use a Malefic)
Fig. 2, the player has moused over the DRG to soft-target them with 'The Spear', which makes sense, as under the proposed reworked effects, Spear boosts Physical damage by 10%
A system like this could be expanded upon with additional functionality too. For example, the first thing that comes to mind is 'other skills'. Esuna, Benison, DragonSight-ing your melee buddy, any of the shareable Tank moves like Nascent or Cover, etc, could potentially benefit from something like this. Additionally, being able to customize the positions of the party members, as you can with the party list, would allow for something like the above examples. This would allow a player to set the positions of each member to their relative 'clock spots' for the fight, which also causes this system to double up as a memory helper to remind the AST 'hey, you're on the East side for Proteans'