Basically, what Jito is onto is pure golden. But I'd like to expand it.
I would like to see all end game content from every expansion to be fit to be end game content for the current expansion. Right now we have countless raids, dungeons, profession items, potions, gear, heck even quests and achievements that are just useless. Sure it's fun to have or do them, but that's about it.
Raid progression would still move with the expansion, with each expansion, each set of raids will be harder, but there would be so, so, SO much more choice in what to do.
With each expansion we will have more crafted gear to choose from.
-How many freaking end game items would we have? Wouldn't it all be the same?
Well, I think with this, either chance on hit: items will make a new return, or stats will just scale up somewhat. As a level 60 player you can go from 60 strength + 70 stamina on plate, to 300 strength + 350 stamina on plate, depending on your tier progress.
-Wouldn't that cause huge unbalance in, let's say, pvp? Wouldn't gear become even more of a deciding factor than it already is?
There are many fixes to this, for example itemlevel brackets, or players would have equal gear (although that knocks the sense out of progression imo).
-Wouldn't new players get confused with how to begin or what to do?
Right now there is the Hero's Call Board to let new players know where to quest. I am confident that a similar system will help new players on their way.
-With so many items to craft, wouldn't players be confused as to what is best?
There will always be people who analyze every single item available thoroughly, and people will offer items that are the best. Guides will be made, but with this, you have more choice, like the current talent tree. I think it would be just as rewarding to play with the items you like most than what is best. There is more choice in items like the stealth potion, who still uses that anyway? Or Cat's Eye Elixir?
-What about Blizzard? A change like this requires a changing the entire game, which will cost a truckload of effort. People may not like the new changes and unsub, causing Blizz to lose millions of players. They would never do that.
Keep in mind, Blizzard has also completely revamped classic leveling content in Cataclysm. I think, if they have their mind set on something, they will do t. Ring of Peace is also the result of that, /lol.
That's all I can currently come up with. Thanks for reading if you did get this far. :P