Karazhan contends for the top spot of "most beloved raid" in all of World of Warcraft with Ulduar. It's also a pretty special place for me, as it's the very first raid I ever tanked (we only got about halfway through on two separate runs and I didn't get a single gear drop, but it's still my first experience of raid tanking.)
It's also a really fantastically atmospheric location. I've often dreamed up "haunted house" style raids, but really, Karazhan already is that.
So it's pretty exciting to see that we will absolutely be heading back there in Legion's first (hopefully of many) content patch.
Here's what we know:
Karazhan as it exists today will be sticking around. Unlike previous revamps, this one is not going to displace the old dungeon (the same will be true, I believe, of the old Violet Hold in Northrend's Dalaran, though that was certainly not as beloved an instance.) This fact alone puts me essentially 100% behind the revamp - nothing should be lost in the process, and we'll be able to go to Karazhan 70 to farm the Fiery Warhorse and all the other stuff there.
We also know that this dungeon will have a massive nine bosses. If you don't count the servant quarter mini bosses, the Chess Event, or Nightbane, that's the same as the bosses in the original raid (the only reason not to count Nightbane is that you had to complete a separate quest chain after beating Malchezaar in order to summon him, making him a kind of proto-Algalon-style boss.)
Additionally, we know that the patch is going to add other things, including a small raid in Stormheim (presumably this will be a 1-3 boss instance in the vein of Throne of the Four Winds.)
Here's what we don't know:
It's possible that this will be a simple revamp like Zul'Aman. Other than cosmetically changing Zul'jin into some other troll, the Zul'Aman dungeon was essentially the same as the raid had been, but with a few mechanics switched out to allow for things like having only one tank.
At bare minimum, I could imagine Blizzard doing something like this as a way to ensure that we're getting new (or "new") content at a steady rate.
However, what I'm hoping for is more of a "sequel dungeon," the way that Blackwing Descent was a totally new raid that borrowed heavily on the theme of Blackwing Lair, or similarly how the Firelands was a kind of sequel to Molten Core. That doesn't mean that we have to have the same final boss (though Malchezaar coming back would certainly fit into this idea of demons being unkillable - though by the time we got to the top of the tower, was the Netherspace not part of the Twisting Nether enough to permanently kill him?)
We know that some classes and specs are going to enter a new area underneath Karazhan to get their artifacts, so I would hope that instead of simply going through the same area we've already seen, we get a totally new part of the tower, with brand new bosses.
The 7.1 PTR is going to go up very soon after Legion proper launches, so if Blizzard doesn't explicitly answer questions before then, we'll start being able to find them out for ourselves.
Still, even in the worst-case scenario, where we just wind up running the old raid in a dungeon form, I'll be eager to do so.