In order to make the game better, here's how I would do the next expansion:
1) Merge dead realms (already happening, sort of, so this is taken care of). This was the most important bit. Can't really play the game solo you know.
2) New character models. This absolutely needs to happen and I certainly hope the models will be done when the next expansion hits the shelves.
3) Dynamic questing. What I mean by this is that Wow needs to utilize phasing and it's shared tagging a bit more. Currently the problem with leveling is that you outgrow your leveling zones too fast. The way to fix this would be that there shouldn't be anymore level requirements for zones. Instead the mobs & quest rewards in the zone should keep scaling with your level (as in all mobs are yellow always). This way you would stay in one zone till the end as every zone basically has it's own story and phasing these days. You would only be able to see (unless you group up) players that are within two levels of you. This way you can level one character in zones a, b and c, but you still have the motivation to level an alt through other zones. In order to reach mach level you would of course have to level through the new expansion zones, so this would only apply to old zones.
4) More exclusive content and less faction bloat. In mop there was just too many factions to gain rep with. Of course gating the only valor gear behind rep was bad and shouldn't be done anymore. In order to reduce faction bloat, they should do more scryer & aldor type of solutions. You have to pick one faction to root for which makes you hostile with the others. This should effect your whole leveling experience and other stuff for the duration of the expansion. This would of course also encourage you to level alts to see what the other side is like.
5) Reputations should be an alternative way to gear up instead of mandatory. Mop did good in adding more gear options for solo players, but those valor rewards shouldn't be the only ones. Instead they should be ALTERNATIVE way to gearing so they don't feel mandatory. In addition there should be more ways to gain reputation instead of just dailies. Bring back dungeon tabards and add possible events that you participate in in order to gain reputation. Just add a weekly cap for reputation gains in to increase the amount of time it takes to reach exalted.
6) Reputations should be account wide. This would help altoholics so you don't need to go through all that hazzle again once you've already done it. The mutually exclusive reputations wouldn't be affected by this however.
7) Bring back dungeons! Gearing your alts through dungeons is a much less painful way than through doing dailies in order to get coins for LFR. The cataclysm purpose for LFR worked much better than the Mop way. Scenarios are just really awful.
8) Change LFR into 10 man mode. The difficulty or relative gear strenght you gain from it doesn't need to change. What this would accomplish is that it reduces to chances of people afking and leeching. Your personal donation for the raid is better felt in a small group. I also think this would shorten the queue times for LFR quite a bit when you don't need to wait for so many healers.
9) Remove PVP gear. Blizzard has tried to make the walls smaller between pve and pvp, but it just isn't happening. The amount of effort it takes to gear up in pvp is just too much if you want to do pve as well. Instead you should just automatically get pvp gear when you enter a BG or Arenas. That way everyone has the same gear strenght and you don't just get steamrolled or steamroll others with gear. This would in my opinion make the ladders more competitive. Cosmetic rewards & vanity stuff & transmogging gear could be earned through arenas and battlegrounds. Maybe some gold and reputations as well.
10) 4th specs for all classes. This would be the "big" thing for next expansion. I don't want new classes or races. Most already have enough alts. Instead they should enhance the gaming experience for those. Examples:
Hunter: A melee spec like survival originally was intended to be.
Warlock: A tanking spec.
Shaman: A tanking spec like enhancement sort of was originally.
Obviously this requires a lot of balancing, but we pay to play this game so we can expect it. Besides, fun has always been more important than balance. I mean, it's game, remember?
11) Reduce button bloat! This needs to happen as some classes like warlocks & healing priests are a pain to play. This can be achieved through these means:
a) Simply remove some abilities
b) Merge some abilities
c) Make more abilities spec exclusive.
d) Replace some old spells with new ones.
12) Make classes feel different again. I realize that "bring the player not the class" was the goal, but it just make the game feel dull when every class feels the same. Everyone can do everything. It just isn't right. A boomking doesn't need mad CC and dk's dont need all the same debuffs as some other class. Game was more fun when your class was actually really needed for something.