The tried and true MMORPG leveling style of grind quests, kill mobs, gain power, unlock new areas, etc STILL WORKS.
The major, major issue with WoW is that they have truly gutted this process for leveling.
They have gone so far towards instant gratification that it has gone OVERBOARD.
Players do not want to grind mobs for an hour for a single level anymore, but they also do not want to blink their eyes and skip past entire content zones.
Wow needs to slow down the leveling process, put emphasis on their better designed areas and quests, give dungeons the fun of loot again, rather than just insanely fast levels such that the loot is pointless, etc.
Let players on their 5th alt be decked out in exp-boost so they don't have to play the same shit over and over again, but allow new players the opportunity to actually get to experience the world.
Wow probably has the largest, depth-filled world of any game, but Blizzard has really discouraged players from seeing anything but the last 1% of it.
I honestly would love to see them drastically reduce baseline exp, and buff heirloom exp.
Slow down the process for new players, but allow the same speedy leveling for repeat levelers. The outcry of those with multiple alts having to do the same thing over and over again, combined with Blizzard putting all their eggs in the endgame basket, has encouraged blizzard to really ruin the game in the levels 1-(max level-1).
As it stands, WoW really begins once you hit level 100 and jump into Tanaan. Think about how crazy that is.... Azeroth, Outlands, and Draenor are INSANELY large and full of detailed zones and dungeons... yet the entire focus on the game is in one outdoor zone, a raid, and a few instances.
Endgame will ofc always be BY FAR the major emphasis of the game, however I do not think Blizzard needs to sacrifice every other element to do that.
If engaging gameplay exists BEFORE endgame, then you will not have to bet everything on that and players will get hooked on the game before having to rush to 100 and grind tanaan gear, etc.