This but semi-unironically. We are doing a Void expansion soon. A cross-over with Legacy of the Void would be cool. Let me get that dope energy Scythe - we already have the energy fist weapons.
And I’d love to get GrandFather and Windforce in WoW. We’ve already had multiple incursions of Diablo into the universe in Hearthstone as well as a limited crossover in WoW proper.
The ability to change idle poses from FF14. The fact they have five different sit and standing animations and some of them are obviously meant for casual settings, while others are meant for other emotions, is fantastic. And the RP’er in me would kill for the ability to do that in WoW.
Both from SWTOR:
Transmog slot, not item.
Player housing.
Some sort of on going invasion system, like ESO’s Dolmen’s. There’s plenty of reasons and locations to implement - scourge, defias, etc.
SWTOR crafting materials storage system… It used to get stored in hold storage that all characters have access to… which you can still do that…
But now they’ve got a materials storage system where collected mats automatically go to, and every character in your legacy has access to using.
Housing system from Wildstar.
…this too.
Swtor housing and voice acting, and the older family tree thing (for rp). Lastly the legacy “talent tree” ie unlock to give 5% more xp
-BDO and BnS character creation depth
-Mogs from (insert any eastern MMO here)
-The ability to swap to another alt without entirely logging off
-The ability to choose your own world phase from a drop-down
Account wide progression like in Lost Ark. It’s easy to switch classes there.
Player Housing from Wildstar.
Capturing keeps from Dark Age of Camelot. Including hanging your guild banners on them, and upgrading them + the NPCs to keep other people from taking it from you.
The developers from FF.
Semper Fi! 
Honestly this. I picked the Sylvari as a joke race and ended up loving the heck out of it. Some of the GW2 classes are worth stealing too.
I really wish we had more vanity driven content like FF14. And maybe more properly aligned normal mode content. Like the new alliance raid that dropped. It’s not easy but it’s not hard. Just requires 24 people (3 groups of 8) to coordinate for pretty darn good gear.
The selection of toys from Looney Toons Space Race to use in PVP just for giggles.
DCUO’s player housing, but with the WoW setting in mind; imagine selecting where to put your house, selecting a preset theme for the inside, and then placing furniture you get from content anywhere you please.
“Deposit all materials” button that send all crafting mats in your bag to your bank tab from anywhere in the world.
I want to say “auto-loot”, but WoW already has that in the game (Torghast) and simply refuses to give it to me. Please…I’ll grind for it. I spend days grinding for it in GW2. I’ll spend weeks grinding for it in WoW. Just please stop wasting my time with manual looting.
AH from GW2
Story, class switching, flying unlock, and community from FFXIV
Voiceover from SWTOR
Combat system from Tera
Player housing from Rift
Dungeons from Wildstar
Gathering from New World
Crafting from LOTRO
Character customization from BDO
Crafting storage system like in Swtor where all crafting materials go into a separate bank that has unlimited storage and is only for those materials. Also like in swtor have it go into that storage automatically when it’s obtained vs your bags.
Swtor housing system where houses are instanced, in a few popular locations and capitals and can be fully decorated with the option of having your house public so anyone can visit anytime.
A class like Gazlowe from Heroes of the Storm.
Bombs, Turrets, and Shredders.
Most MMOs have things I Dislike more than Like.