Well, color me surprised: it turns out that 7.1's Return to Karazhan patch will release in a mere eleven days. As a quick recap, here's what to expect:
Return to Karazhan:
We're getting a Mythic-only 5-player dungeon at Karazhan. To be clear, this is not replacing the raid. The entrance will be elsewhere on the tower. This dungeon has nine bosses. Some of the bosses will be familiar, like the Opera Event (though this gets new "plays,") Maiden of Virtue, Moroes, Attumen the Huntsman, and the Curator. However, the second half of the dungeon will be very different, with new bosses and an upside-down version of the Library. The gear rewards will range from 855 to 860, as this dungeon is intended to be somewhat more of a challenge than other Mythics.
The Karazhan dungeon will also have a set of quests in Deadwind Pass that will allow players to attune to the dungeon, though I'm given to understand these won't be terribly time-consuming.
Trial of Valor:
This is a three-boss raid that has us first prove ourselves again to Odyn, then march into Helheim and take down the three-headed Helhound Guarm and then finally slay Helya. The raid will not open up immediately, instead there will be a two-week delay after the patch goes live. I'm still not sure what the gear rewards are for this.
Suramar Campaign Quests:
New quests, probably unlocking on a weekly basis, will show up in Suramar to lay the groundwork for the Nighthold raid. Given that the raid is technically-sort-of a 7.0 feature, we shouldn't have to wait for a patch to get it opened, so Nighthold could arrive at any time (though I'd imagine that they'd give Trial of Valor a month or two before Nighthold opens.)
Tons of Other Changes!
As with any major patch, we're going to see plenty of class tweaks and such. I believe there's going to be a Blood of Sargeras vendor to whom you can trade blood for other crafting materials.