Yes they can keep expanding their universe, but you can't expand it 5 minutes before you consume it.
Blizzard need to be developing long-term heroes and villains, especially villains right now, to make the world compelling and unpredicatable.
Look at Yrel, she pops up out of nowhere at the start of WoD, and then disappears at the end of the xpac.
It was the same with the Orc clan leaders, Blizzard had to work really hard to build them up as villains as quickly as possible before we promptly knocked them off.
Can you imagine every xpac being like WoD?
The fact that we care so much about characters like Arthas, Jaina, Anduin, Illidan etc are because we've been reading their story and fighting side-by-side with them for years now. They're complex individuals.