Since Warlords of Draenor received an overwhelming amount of criticism over its complete and utter lack of content, Blizzard has decided to start off the Legion expansion on a more positive note - by announcing a massive content update a mere month after release.
Patch 7.1, or rather the Return to Kharazan, will bring with it Kharazan as a challenging five player dungeon, a brand new Trial of Valor raid, and for those that prefer the open world questing experience - a series of missions and weekly quests based in Suramar. If you're wondering what all of this looks like, here's a recently posted video preview, as well as a brief overview of what exactly you can expect from each of the new features:
Once released, Kharazan is going to be the most difficult dungeon in Legion, far more challenging than your standard Mythic dungeon. Naturally, this also means its going to be rather generous with epic loot, so there is certainly an incentive to put on your tryhard pants and give it your best. If you do end up failing, however, worry not as Kharazan is going to be on a weekly lockout, so you'll have plenty of time to face off against the new chess encounter and opera sections.
On the other hand, the Trial of Valor raid is going to be a bit of an intermediate piece of content as it fits between the two previously announced raids. As for the raid location, you will be heading over to Helheim for this one in order to bring down Helya, the annoyingly smug guardian of Vyrkul hell, and Guarm, her gigantic, three-headed canine guardian.
And finally we have the Suramar offensive where we will be taking the fight to Elisandre and the Legion in order to recapture the Nightwell. As you might expect from any open-world area, there will be plenty of new missions to undertake and enemies to whack over the head, but also a whole bunch of weekly events that will slowly progress the story, eventually culminating with what I assume is our full control over Suramar.
That is unfortunately all we know about Patch 7.1 right now, but on the positive side its going to be arriving to the PTR (Public Test Realm) at some point in the near future. Until then, have fun with the seemingly limitless content Legion has on offer so far.