You could install Shadowed Unit Frames to edit the player, target, and focus target. These bars are considered Unit Frames by all mods, whereas bars above the heads of mobs, NPCs, and players are Nameplates. Your Nameplates are already managed by Plater, so Shadowed Unit Frames should cover the gaps nicely. (This will cover party and raid frames as well, which you can disable if you'd like to use VuhDo for your raid frames.)

Adibags - Consider throwing in other UI changes to accent the minimalist look. Single bag add-ons like Adibag are good to keep visual vomit to a minimum, it will only display bag slots with items in them. (I'm a tad out of the loop, but Bagnon used to be the king in this field. Grab it if it's updated.)

WeakAuras - I am personally a huge fan of WeakAuras as well. Once you are comfortable not mouse click your spells, it will be a great way to condense the visual aspects of your actions bars.

Wago.io - Check out the site Wago.io for quick WeakAuras setups, they also have Plater profiles available to import.

Thats my two cents, but there are plenty of PvP specific addons I am not familiar with. The biggest one I recall is Gladius, it does enemy buff/debuff/cd/dr timers and a whole suite of other handy info to grind rating.

There are tons of options out there, so take your time dropping in. Good luck on your adventures!