The topic is what we will probably see in the next expansion based on evidence, lore, and posts by blizzard employees already. Make a suggestion, provide your evidence, and list pros and cons if there are any. Over time I will update this and bump the most likely theories to the top.
TL;DR: This is not for baseless speculation or to just say what you hope it is. It's what evidence exists already. This post will be continually updated as more evidence comes to light to reflect the most likely candidates.
Theme:
1. Elemental Plane of Water/Azshara Theme
Evidence in favor:
+Naga storyline has never been finished and they have close ties with the Legion
+Underwater mounts that are currently unused.
+N'zoth Storyline never finished.
+Lots of Coastal Zones never fleshed out (Northern Eastern Kingdoms, Kul Tiras, and Tomb of Sargeras)
+Azshara still at large.
Evidence Against:
-Vashjir was largely unliked by most players (and so are most underwater quests).
-Naga have always been a subplot.
2. Another Burning Legion Theme
Evidence in favor:
+Wrathion conversation shows green meteors attacking Azeroth.
+The hint of a coming Army of Light.
+Could give Outland the Cataclysm overhaul, smoothing out the content between expansions.
+Chris Metzen says he wants to go to Argus.
+Chris Metzen hinted at an Illidan revival.
Evidence Against:
-This could potentially "end the story" too soon, if Sargeras is to be considered the final bad guy of WoW.
-It's implied the Army of Light would unite the horde and the alliance. Doesn't look like that will be happening anytime soon. (Unless killing Garrosh brings peace)
3. Emerald Dream
Evidence in favor:
+Emerald Dream plot has come and gone with little conclusion in-game.
+Many fans want to know more about it and it's been teased since Vanilla.
+We've seen a lot of places named (the Eye of Ysera, Rift of Aln, Daral'nir). Not including the few datamined from Classic.
Evidence Against:
-Plot hasn't been touched since early Cataclysm
-Very Druid specific and hardly relevant to other classes.
-Most of the Nightmare plot was resolved in Stormrage by Richard Knaak
-Content made during Classic has yet to even be used.
4. Something Else Entirely
+There are a few worlds mentioned or visited in WoW that aren't Azeroth, Draenor, or Emerald Dream.
Zones/Continents:
1. Outland Revamp
(Or another Outland Continent)
Evidence in favor:
+Blizzard has admitted to wanting to redo Outland when it feels right
+Older maps of Draenor showed that the continent continued beyond what we currently see
+New Warlock Quest is set at the Black Temple
Evidence Against:
-If the legion comes to Azeroth, a revamp of outland isn't needed.
-It would be the first time an expansion was a remake of an expansion.
-Last time they did a revamp of a continent it was considered a flop.
2. More Landmasses in Azeroth
Evidence in Favor:
+A few unused zones still
+Some locations mentioned in Lore, but as yet unseen (Kul Tiras, Kezan/Undermine, Tel'Abim, just to name a few)
Evidence Against:
-Too much like a cataclysm overhaul
3. Underwater Zones
Evidence in favor:
Same evidences as the Plane of Water themed expansion.
+Could be combined with the "More Landmasses in Azeroth" idea
Evidence Against:
-Vash'jir. Anymore underwater zones would have to be designed with a different layout.
Races:
Horde Theories:
Ogres:
+Metzen has mentioned before he would like to see Ogres as playable someday.
+Ogres have long been members of the Horde.
+Many horde players want to see them as an option.
+A revisit to Outland would boost chances of them being used.
Hozen:
+Already allied with the Horde
-In small numbers, however.
Alliance Theories:
High Elves:
+Players have wanted them for years.
+Already members of the alliance.
+Would add another Warlock and Paladin race to the Alliance.
+Would be the first non-monster/animal race since Classic.
-Re-use of Models would probably get some backlash.
-Alliance already have an Elf race
-Very low populations
The Broken:
+Would fit a legion/outland expansion
+Could be the race Alliance gets of Horde gets Ogres
-Might be more of a Shaman-Subrace of Draenei
-Draenei lore was originally based on the broken.
Jinyu:
+Already allied with Alliance
+Connection with seas would fit an Azshara expansion
Neutral Theories:
Sub Races:+There is already a lot of lore in places for sub-races to exist. Mag'har, Taunka, High Elves/Highborne, Dark Iron Dwarves, that are all technically members of the Alliance or the Horde.
Blood/High Elves:
+With Pandaren, making the Elves into a neutral race is now a possibilty, with faction choosing becoming the determining racial name
-This would mean the Horde "lose" a race
Naga:
+Wanted by players for years.
+Would fit an Aquatic themed expansion.
-Only one faction not loyal to Azshara.
Ethereals:
+Fits with a legion themed expansion
+Fits a Demon Hunter class
-Very Neutral Race
-Don't wear armor
Classes:
New Mail Wearing Class
If there is a new class added to the game will very likely be a Mail wearing class. It would either be a hybrid class of DPS/Tank or DPS/Healer. We have 4 pure DPS classes and 3 3-spec Hybrid classes. It cannot be a very race-specific class.
Like Monks and Death Knights there will probably be a reason for every race being able to roll one. After exempting Pandaren from DK's, and Worgen/Goblins from Monks, I don't think Blizzard will continue leaving races out of classes. They might. But I doubt it.
There's also a good chance it would be a ranged class. Currently hunters are the only class that uses guns and bows.
I'd also like to stress that there's a very good chance we won't get a new class at all this next expansion. They waited one full expansion between Death Knights and now to add one, they may do it again. In Q&A's they said Death Knights were a head ache and they weren't sure if WoW even needed a new class. With Monks, Blizzard may decide they're done adding new classes.
Regardless;
Evidence:
+This will bring the classes up to 3 plate, 3 mail, 3 leather, and 3 spirit.
+It would fill the gap removing Shaman-Tank created.
+They could use gear that Shaman use and hunters once used.
Candidates:
1. Demon Hunter (DPS/Tank)
+A monk at the peak of serenity claims to have always wished to be a demon hunter. (This could be a joke)
+Another Legion themed expansion would be a good time to roll this out.
+Would fit the description of using Mail, and being a DPS/Tank. Might finally be the "warlock tank"
-Hinges on it being a Legion themed Expansion
-Warlocks already have a lot of "Demon Hunter" rolled into them.
Professions:
Archaeology was added in Cataclysm and Cooking received a major facelift in Mists of Pandaria. There will likely be another new profession or a minor profession like Fishing or First Aid will receive a "Mastery" like Cooking did in Mists of Pandaria.
1. Woodcrafting/Woodcutter
+This item could be a joke, but regardless: An item hinting at +15 Woodcrafting.
+Could craft staves, wands, and bows. (filling gaps that other professions have just touched on)
+If player housing is ever implemented, could craft furniture.
+Could add a whole new gathering resource to the game.