Im gonna tell you why a light crusade exp would be garbage in a lot less words.
A light Crusade would would mean we are dealing with more cosmic level entities. WoW’s story sucks when it comes to its Cosmic Lore, and largly has been rejected/not enjoyed by the community. Source on that being the reception of SL and the respecitive realms.
The reason wows cosmic lore is garbage is because before SL, wows cosmic lore was pretty straight forward, you had light, shadow and reality which was the 2 of those combined, and you had the elemental planes, that was really it, it was simple, it worked, and everyone was fine with it.
Now blizzard is trying to make more cosmic lore, while at the same time, expecting the player to just accept anything and everything they throw at them, and that they will love it. When in reality, you get the following happening.
Light crusade would just be more cosmic level crappy story that no one really wants.
The best thing we can get for an exp would be going back to azeroth and dealing with more azeroth problems. There is far far more to do there then going into other realms. Here is a great list of things that we can still do:
South Seas exp (Give us a fun ship mechanic that we get to build.)
Exploring the other side of the azeroth
Caverns of time exp (Going back to make sure the time line is set right, and preserve it)
Narubian empire (Let us explore ancient and old narubian empire under the earth, something with the dwarfs would be great as well)