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.
- Buffs, Debuffs, Cooldowns... Raven or the lighter weight Aura Frames
- Action Bars and Keybinds: Dominos Action Bars, Macro Toolkit, OPie, AutoBar, BindPad, Clique and/or Button Forge
- Masque to make icons look pretty
- OmniCC to create cooldown counts and sweeps.
- Information Overload: Bazooka and Broker Everything
- Immersion if you want to focus on reading the quest text more.
- Gnosis cast bars
- Map: Mapster, Cromulent, HandyNotes
- SexyMap to make your minimap sexy.
- Raid/Party Frames options: Grid2 or VuhDo
- Scrolling Combat Text: Miks Scrolling Battle Text or xCT+ Combat Text
- Quest Tracker: Kaliel's Quest Tracker or Sorha Quest Log. Useful with TomTom.
- TipTac for tooltips
- TransmogTokens, TokenSplainer, and Relic Inspector take mystery out of some items.
- Mail: Postal and/or Inbox Mailbag
- Nameplates: Kui Nameplates or TidyPlates
- Chat: Prat
- Unit Frames: Shadowed Unit Frames, PitBull and maybe Stuf are three popular options.
- xLoot for pretty loot frames
- eAlign to help with UI element placement
- AdiBags is hands down the best bag addon currently available.
- Scrap to automate dumping stuff at the vendor. More than just greys.
- Scrap Cleaner auto-deletes least valuable scrap when bags are full.
- New Openables never miss an open-able item in your bags again.
- Outfitter is a more powerful version of the now-built-in equipment manager.
- Pawn helps evaluate upgrades. (Designed mostly for level-capped players.)
- SimPermut and/or Simulationcraft are for exporting your character information for import to http://www.simulationcraft.org/. Used to generate your own stat weights which can be imported to Pawn.
- Advanced Interface Options exposes a lot of hidden or removed options.
- Better Addon List does what it says on the tin. Useful if you end up with a lot of addons.
- Leatric Plus adds a whole bunch of small quality of life changes.
- Silver Dragon since you like killing those rares. Better than NPC_Scan.
- ErrorFilter White-/Black-lists for error messages.
- Altaholic or Armory can help you keep your head around many alts.
- BadBoy and plugins spam blocker.
- BlockChinese is a popular chat filter on Illidan.
- BlizzMove unlocks a bunch of unmoveable UI frames.
- LiteMount makes mounting easy.
- Server Hop NPC you're looking for is dead?
- Cork has a lot of premade visual alerts that some people find useful.
- Rematch Manage pets and teams plus many other features.
- PetJournal Enhanced improves the Journal interface.
- PetTracker is your addon for the wild pet battler/collector.
- Rarity gives info in tool-tips for collectibles dropped among other things.
- Battle Pet Breed ID for the min-maxers.
- BattlePetCount shows you info on pets you already own.
- Collection Shop for searching the AH for collectibles that you need.
- Overachiever and Character Achievements for the achievement junkies.
- Auctionator is the most popular AH UI overhaul.
- The Undermine Journal will add AH values to tooltips so you don't trash 50,000g pets anymore.
- TradeSkillMaster (TSM) is the suite of very powerful addons for the dedicated profession/AH goblins.
- TradeSkillInfo Look up any recipe and adds tooptip info for mats.
- Skillet crafting UI overhaul.
- Ackis Recipe List Find missing recipes.
- Nomi Cakes Shows you which Legion Recipes Nomi has yet to give you.
- GatherMate2 plus Database displays resource nodes on your map.
- Routes plots a route of the zone to look for gathering nodes.
- FishingBuddy for the anglers.
- Easy Disenchant
- Easy Obliterate
- Archy and Archeology Helper for archaeologists.
- Deadly Boss Mods (DBM) or BigWigs and LittleWigs. Search Curse to get the extra/legacy modules.
- Saved Instances displays lockouts.
- Dungeon Difficulty so you (hopefully) notice you're on the right difficulty.
- Incentive Program alerts when there are extra rewards offered for your role in the group finder.
- Premade Group Filter powerful filters for the premade group finder.
- Details! for damage meters.
- Talent Set Manager save and quickly load talent sets.
- GTFO audio alerts when you're standing in the ouch.
- GottaGoFast is a nice replacement Mythic+ Dungeon timer.
- Exorsus Raid Tools is a bundle of really great tools that replace quite a few common raid addons.
- Lost Control alerts others when you're cc'd (not super important for DPS)
- Weak Auras 2 is the successor to Power Auras. Many raid leaders have prepared auras to alert raid members of certain boss mechanics, etc. These can be exported/imported as base64 alphanumeric strings and shared. It is worth learning. I put it here rather than other UI sections because it is often mandatory for some raid groups. It is worth learning how to create your own auras as well. There are some websites out there where people share auras they've created (1) Wago.io and (2) Weak Auras Online but you'll also notice on wowhead, under certain pages like spells or auras, there is now a button that will give you a basic icon aura string that you can import to weak auras. A simpler alternative to WA is TellMeWhen.
- Personal Loot Helper to be a bro and hand over gear that a teammate can use. Blizzard added the ability to trade soulbound loot with other players from your instance within a time limit. If you (or someone else) loots an item, it checks everyone's slot and alerts you in chat if it is not an upgrade for the looter, but is an upgrade for someone else. It has the option to broadcast this to everyone, but that can be a bit rude. I use it simply so I can offer upgrades to others.
- BattleGround Targets unit frames for BGs
- GladiatorlosSA2 audio alerts. Recommend just target/focus.
- Spy for alerts to enemy faction presence.
- REPorter replacement battleground map and communication tool.
- Gladius arena frames
- OmniBar enemy CD usage/availability display
- Appearance Tooltip shows what an item will look like and if you've collected it.
- CanIMogIt if you want more obvious icons and tooltips.
- mOnArs WardrobeHelper as a mob drop farming guide.
- World Quest Group Finder helps automate the process of finding, creating, and joining groups to knock out world quests quickly.
I recommend choosing only one of the following so filters work as expected:
- World Quest Tracker is the most popular.
- Angry World Quests I find better for rapidly switching between displaying certain types of quests.
- World Quests List has great sorting and filtering plus some nice helpers for some of the puzzle quests.