You know how it will end, the Titans will see the world of Azeroth in chaos, will try to reboot it, and lead to us fighting them for our survival. A few twists will happen, maybe involving Sargeras making a return and killing the Pantheon, leading to us winning and the world being safe for the next expansion when they make Hogger the next great enemy.
Oh, as for the Old Gods and Azeroth being destroyed. I think the Titans learned that at least one of them has to live in order to keep the core of the world from destabilizing, and so killing C'Thun didn't do anything, and killing the other Old God didn't do anything, but the last three were imprisoned because it was to risky to leave only one alive which then could die somehow taking the world with it.
That might be a plot point after we kill Yogg and whatever other Old God they throw at us, leaving the last one being all "haha you fools! You kill me you kill yourselves!"
P.S. Now that I think about it, this makes the quest in Shadowmoon Valley a lot more clear. If the Old Gods do act by moving and infecting planets, then Outland was a logical step for them to move due to its importance as a hub. It also explains why the Naaru talked about them as trying to conquer the universe, maybe Azeroth was just one of many planets infected with Old Gods.