People are suggesting complex ways to make the game more immersive and more enjoyable. I don't believe that. I think the method to make WoW more immersive is actually very simple.
1. Restore class specific quests
Few things were more immersive than me being a Shaman and going around the world to commune with the elements. Granted some quests were tedious, but speaking with the various elementals really connected me to my class and made me feel special, and part of a larger world. Blizzard really needs to restore these aspects to the game.
2. Race specific quests
If I'm a Night Elf, I'd like to feel like my experience in the game is unique only to my race. You definitely get some of this in isolated starter zones, but I'd like to see it spread throughout your leveling experience and even at max level.
3. Race specific abilities
When I first played as a priest, I thought it was pretty cool how Night Elf priests got Star Shards and Forsaken got Plaque. I think Blizzard should bring back this feature, but only make the differences cosmetic. For example, you can have the spell do the same thing, but just have a different icon and animation. Sort of how Heroism/Bloodlust works among Shaman. This would give each race a bit of flavor, even if it really doesn't mean anything on the DPS meters.