Since MOP (or agruebly, Cata), WOW has always connected the ending of one expansion to the next in some way.
Cataclysm (If counted)-The Cataclysm cleared the mists around Pandaria, which allowed the Alliance and Horde to reach it.
MOP-Garrosh’s arrest led to him escaping to Alternate Draenor, where he helped form Iron Horde and plotted to invade Azeroth.
WOD-Upon his defeat, Archimonde sent AU Guldan to Azeroth to jumpstart the Legion invasion there.
Legion-Before he was sealed by the Titans, Sargeras stabs Azeroth with his giant sword, causing Azeroth’s life to be threatened. In addition, Azerite is discovered and it kicks off the next war between Alliance and Horde.
BFA-Slyvannas is exposed as a traitor and she flees to Icecrown to defeat Bolvar and shatter the Crown of Domination, opening the doorway to the Shadowlands.
If tradition continues, something that happens in the ending of Shadowlands will lead to the next major event in WOW, leading to 10.0.
I’ve mentioned this theory before, but here it is again. My theory is since the Jailer is connected to Torghast, his defeat will cause it to weaken, causing a prison break of sorts.
As lore states, Torghast is full of the most evil souls that ever existed amongst the cosmos. These souls were so evil and irreedemable, nothing Revendreth could do would turn them over (remember, one of the Venthyr was a world-killer, and even she was rehabilated). They were locked up and tortured forever.
But with the Jailer defeated, their seals broke free, and those who still had the sense to will escape. They’ll flee Torghast and across the cosmos, not just to Azeroth, but to the various planes of magic and such. Among these souls could be Arthas himself for a future storyline.
This could be what kicks off the big elemental war, as the balance is broken with various souls strengthening the others over. But I think it would be better to do it over time; the next expansion could deal with one such escaped soul, and after it, we discover the further effects of Torghast’s souls escaping.
One such example could be an evil dragon soul (we could use Deathwing, but maybe a different character would be a better idea) flees to Azeroth, and plans to use the power on the Dragon Isles to revive themselves. This not only threatens Azeroth, but said power could be used to save the dying Dragonflights and will be lost if the evil dragon gets to it first. This could be why the players go to the Dragon Isles.
Another tie-in to 10.0 or another expansion could be Sire Dentharius. As of the Korithia storyline, the Dreadlords have saved him from imprisonment and since he is not confirmed to appear in 9.2, he may be buying his time for his own agenda. He may have not been completely loyal to Jailer, but was hiding behind his own threat for his own plans.