This is partly based on a fan theory, but this would be an interesting direction for future Warcraft expansions to take.
The recent Warcraft Chronicle book gave us more information about the Void Lords and how they are the true main antagonists of the Warcraft universe - a fear of the corruption they can bring about is what caused Sargeras to create the Burning Legion, as he felt that the only way to stop the threat from the Void Lords was to destroy all life in the universe.
What the Void Lords do is send their minions (which we know as the Old Gods) hurtling through space in search of planets containing Titan souls - for lack of a better term, some planets are essentially Titan eggs - of which Azeroth is one. The Old Gods are attempting to corrupt the Titan, which is why it is important that we kill them.
Now we are way too weak to defeat the Void Lords, as is Sargeras and the Burning Legion. However, even though we are at war we do have the same end goal. This is where the fan theory comes in - what if we were to team up with the Burning Legion to take down the Void Lords? One way I could see this happening is if we can finally put an end to the Old Gods on Azeroth, and help to birth the Titan soul incubating inside the planet. Perhaps this Titan could defeat Sargeras and install itself as a new leader of the Burning Legion, one committed to bringing an end to the Void Lords rather than destroying all life in the universe.
Now recently we have seen that Bolvar is active and still keeping the Scourge contained. What if Bolvar could also lead the Scourge into battle?
Imagine an end game in Warcraft where the armies of Azeroth unite with the Scourge and wage war on the Void Lords with the full might of the Burning Legion and the new Titan fighting with us.
Blizzard could very well take the game in a wildly different direction, but this is the way I would do it personally.