As you know, I believe the big problem with WoW is lack of in-game community. What does that mean for leveling? I think the "improvements" to leveling over the years have gotten it all wrong. To truly improve the leveling experience, make it more social. How to do that? Let's borrow some ideas from Diablo 2 and update them.
1. Diablo 2 had a system of runes that could be used to craft amazing gear. One of the best sources of runes was actually from leveling. Every time you took a new character to the hellforge on hell difficulty, you got a random rune of good to very good quality that could be used to build equipment for your main. How does this translate into WoW? I say revamp leveling to be completed multiple times on the same character. Let's say you have a 120 Priest. Toggle a leveling switch on , and it is set back to level 1 and can restart the leveling process. Toggle the switch again, and you are set back to level 120. Progress is saved. So if you level from 1-13, then toggle it off, then 5 days days toggle it back, you are level 13 again and can continue. If you reach level 120 in leveling mode, you get to choose form a suite of rewards, such as a very valuable piece of gear for life at 120, or maybe a collection of mats, or gold, etc. Then you can start back at level 1 the next time you want to level again. This puts more people in the leveling system and helps build a community.
2. Diablo 2 had a system of special builds that were helpful in assisting other players level. The Enchantress Sorc for example, if kitted out with the correct set, could enchant the weapon of new players to deal tremendous damage. We could again borrow this system. Get rid of heirlooms and xp pots because those don't encourage social play. Instead provide special buffs that level 120 players can bestow on low level players to grant them xp boosts or damage boosts, and you want to do it in such a way that it encourages people to help each other and work together (because that's what its all about).