A feature from another MMO I would LOVE to see in WoW
WoW has always and will always use the best innovations that somehow mesh into the game, and adapt them. This is a good practice, whatever some may say about originality and copying, and has made the game better for it.
There is one feature from a certain MMO, from years ago, that was never really brought up and I do wonder why, because I loved it to pieces.
The game in question was/is Lord of the Rings Online, and the element I refer to are the Legendary Weapons/Items.
Now disregard the name "Legendary" for a moment as I explain the concept:
There are class-specific weapons in the game that drop randomly (in WoW, they would drop off bosses ofc), some of the better ones of specific bosses. These weapons have a certain guaranteed amount of base stats and a guaranteed number of class-specific boni. They have a pool of class boni they can roll, and roll a random value of their min/max stat.
Now here comes the really interesting part - the weapon gains part of your experience as you go. You level this weapon, and at certain item levels it gains an additional class ability, again from a pool of boni it can roll from. Additionally, you can upgrade certain boni with runes you can craft.
While some would abhor the amount of time invested into finding the "perfect" weapon with the optimal stats for your spec (which you wouldn't know until you finished leveling it to unlock all possible boni), it motivated me immensely to play the game and was the main reason I stuck with it for quite some time.
To put it in WoW terms, let's assume the following:
Class-specific personal drops off certain bosses in a tier (similar to heirlooms off Garrosh, at a similar drop rate)
They start out with 3 boni, such as
min stat roll: base difficulty ilvl (like 553 on normal)
max stat roll: warforged ilvl
+ x% Bloodthirst/Mortal Strike damage
+ x% Bleed damage
+ 0.x% Rage gain
Levels up to level 30, every 10 levels it gains one more bonus, like from these pools:
+ cooldown reduction on ability x
+ change effect on an ability like a glyph
+ cosmetic effect on weapon/ability
Additionally, you can name the item at max level, and for the cost of either mats or gold or Valor points, can reset the leveling status to try and roll 3 new boni instead of the ones you got, if you're happy with the 3 base rolls.
I personally love this system, it appeals to the item grinder in me (huge Diablo nut too), but I know that many people wouldn't like this kind of system. Also it would probably be too complex and confusing for casual players.