First off, I know you like to keep your UI as vanilla as possible, so I know this one probably won't be your thing. That said, this complete UI replacement is so popular that I should mention it. It is called UlvUI. Download it here: https://www.tukui.org/ (not the TukUI download.) Rather than take several paragraphs to explain all of the changes, it would take less time and have more impact for you to simply install it and fire up the game to see it first hand. If it isn't for you, simply delete it from your addons folder.
For the complete experience, I recommend also grabbing AddonSkins and ElvUI Shadow & Light on Curse.
If you use this, many of the following recommendations become redundant.