Lets start if facts.
Burning Legion = Demons
Diablo = Demon
Scenarium:
1) So in the end of the expansion we will defeat the Legion or push them back.
2) Azeroth is part of a multiverse as seen in WoD with going to alternate realities.
What if as a last ditch effort before being thwarted , Kil'Jaeden speaks of a mythical demon with powers beyond his own. He then summons Diablo from a different universe into Azeroth.
Instead of listening to Kil'Jaeden, Diablo kills him with ease and takes control of the Burning Legion. He also brings lieutenants from his universe.
We now have a new and fresh foe.
The Diablo games do not need Diablo anymore. he has been used up. so instead of letting him fade away, better to use him in more sensible and exciting ways.