If it is to succeed and create potential hype on a third one, it should be about the events leading to the fall of Lordaeron and probably include some Burning Crusade hints/influence into it..
Final scene..... Arthas walking into Lordaeron, on his way to..."succeed" his father. Film ends on the echo of Arthas words.
Now everyone is losing their minds knowing what the next film is about You now know that people WILL go to watch the next one 100% too.