No new class. There are already more than enough. I wouldn't have an issue with shadow hunter if it was added though.
No new race. There are already plenty.
No player housing unless it is just kinda cosmetic stuff. Garrisons were a pretty bad thing for the game.
WoW is an amazing game but there are areas that have been neglected, there are story lines that are a cluster fuck but there are a TON of really good ones. I would love to just see a polishing of everything that is happening. Then use something like the Old Gods as an excuse for people to go an check out everything that has been polished and tweaked around AZEROTH for the opening period of time in the expansion (yay no leveling cluster fuck). Let us go through and discover what the old gods are up to in the first couple weeks, have us provide "reports" on where we should send people and prepare, then a couple weeks in have the Old Gods just come in and start mucking stuff up around the world in different places. Have us scramble around to try and protect the world. MAKE IT FEEL LIKE WE ARE UNDER PRESSURE AND LOSING ZONES. All with the benefit of updating the world, polishing the classes, polishing the races, polishing the storylines and lore. Just make things more cohesive.
I want to see the world re-scaled from 1-110, so you can level anywhere in any expansion at any level between 1 and 110.
A few months in I want the world scaled to 120 or whatever they bump the level to, and allow the completion of old world quests for max level credit. That way you can level ANYWHERE. There is SOOO much content that most of the player base has not experienced. In fact if blizzard released no new content and just current old content in a way that was accessible, most players would have content they could do they have never seen. And you almost never see the higher level expansion zones. Netherstorm/Shadowmoon valley in TBC, you don't do Ice Crown in Wrath, you don't do Uldum in Cataclysm, you don't do half of Pandaria which is breath taking, you don't do half of WoD where leveling was actually really well done, you won't do half of Legion when you are leveling so Suromar, no broken shore, no half of the zones.
ALL of that content can be brought back and it would give people things to do for a really long time. There are a TON of amazing zones out there that are simply less accessible so no one sees them and leveling through dungeons is so easy and mindless.
I want to see stats rescaled. Let's face it, it is now pretty out of hand and it is going to continue growing exponentially which means we are looking at doing BILLIONS in damage. Yeah people will complain "BUT I LIKED SEEING A MILLION DAMAGE" yeah and you used to like seeing 100 damage, and 1,000 damage, and 10,000 damage, and 100,000 damage. And guess what you will like seeing 10,000 or 100,000 damage after a week or so. I would like to see 1-110 content top out at maybe the end of Wrath or Cataclysm level scaling.
More than anything I want to see that. I am willing to bet it will hit more than a few nostalgia buttons for older players, who have done the content but now are onto a completely different mains and get to see this content again. I am willing to bet newer players will like being able to connect more with the rest of the wow stories and lore.
I want them to focus on the classes we have. Let's face it some classes have some serious issues. We don't need another classes when there are classes that really could use attention. We don't NEED any new class or spec: Tanks 6 classes/specs; Healers 5; Ranged 10; Melee 13.
If you want to introduce a new class introduce shadow hunter/shaman, 1 healing spec, 1 spell focused ranged, and 1 physical/poison/cc focused range.
Healer: Blend together shamans and priests, uses wards (basically totems but not elementally), uses shadow heals (basically things similar to shadow mend and using shadows to absorb things and buff allies and debuff enemies. Add in a little linking people's souls together whether enemies or allies.
Ranged spell caster: Again kinda drawing a cross between shamans and priests maybe with a little bit of hunter mixed in there. So things like hex show up, things like fear show up, dots show up, calling primal spirits rather than elementals/pets.
Ranged physical damage: Cross between hunter and rogue. Mobile, shifty, uses poisons, traps, shadow magic, pops into melee range occasionally but stays ranged the majority of the time (shadow step + disengage).
No races I just want them to update current races. Give some variability, not every single horde race needs to be hunched over for instance. Give more customization of current characters (scars, eye patches, sub races...).
Also NO LEGENDARIES for everyone, that was a stupid poorly designed system.
There is so much quality content in the game as it stands, that more than anything it needs to be refreshed to be more accessible. Get the pet battle masters more accessible. PvP needs some serious love. Not new battle grounds how about actually balancing existing battle grounds.
Outside of that have it be old god focused, this would provide the excuse for revisiting older places, have events take place ACROSS Azeroth rather than just in the highest level zones. Have a resurgence of old gods in zones where they were in the past, and zones they would influence (naga invasions around Azeroth).
I want this expansion to be the WORLD of warcraft...not the world of broken isles...the world of draenor part 2...the world of pandaria...the world of oh wait this one actually was the world, but really could have used some higher level activities in the world to interact with the changes I was a little disappointed there were not quests to check out the changes around the world, I digress...the world of Northrend...The world of Draenor Part 1.