7.0 is on live servers now. We're down the rabbit hole, people!
So far, the pre-patch event has not really kicked into gear. Doomsayers can be found across Stormwind and Orgrimmar, but the Broken Shore quest and certainly the demonic invasions have not yet begun. Demon Hunters are, likewise, not yet available, but the twelfth character slot is, so if you want to reserve a name for your Demon Hunter (and maybe try out some Night Elf/Blood Elf models to get a general sense of how you want your DH to look,) now is the time to do it.
I've got two names on reserve on my main Alliance and Horde servers. My Night Elf will be named Veranos, and my Blood Elf (who I have to imagine will be waiting for quite a while to get picked up) will be Calsaran. I didn't have any V- or C-names, and I'm fairly happy with these ones.
With the pre-expansion event slowly ramping up, what should you do now?
Mainly I'd go through your characters and set up your action bars and pick out talents. There's also a Burning Crusade Timewalking event now, which should be a good opportunity for you to re-learn your specs. (Major impression for Protection Paladins - we're a hell of a lot squishier, but I think that's all tanks now... hopefully. Major impression for Subtlety Rogues - this is the spec I have wanted my Rogue to be since vanilla.)
Another major thing is to activate appearances. If you have BoE gear, equip it to unlock the appearance. Note that it still has to be appropriate to your class (in the absolute barest sense - I was able to get the look of an agility staff on my Warlock,) so if you have some leather shoulders on a mail character, send those to your Rogue, Monk, or Druid.
NOTE: if a piece of gear is not for your class, it will not tell you whether you have unlocked its appearance yet, so be sure to shuffle those things around your alts.
Once that's done, sell off gear you're never going to equip again. You'll make lots of gold.
Also, if you have the materials or can afford to buy them, get an Enchanter to make you Tomes of Illusion to unlock weapon enchant appearances. Most of the recipes for these are available from the Enchanting trainers. The Draenor one is purchaseable for a Secret of Draenor Enchanting. There are also two hidden recipes. Tome of Illusions: Secrets of the Shado-Pan is available from the Shado-Pan quartermaster in Townlong Steppes if you're revered with them, and Tome of Illusions: Might of the Elemental Lords is apparently received as a reward for a questline given to you by Therazane at revered (at this time, I've only tried talking to her on the PTR, and she did not have the quest, but I haven't tried this out on the live servers.) Again, this quest rewards the Enchanting recipe, not the item itself.
If you have lower-level alts, be sure to log onto them. If you started playing before Cataclysm, you've probably got a ton of quest reward appearances you never unlocked, so be sure to log onto characters that quested up through the Cataclysm 1-58 experience.
A ton of items have become Toys, so be sure to go through your inventory and learn those or get rid of duplicates.
And finally, remember that every single time a new expansion comes out, people claim their class was ruined by the changes. I've been to this rodeo six times. You'll get used to the new rotations. You might decide you like one class better now than another you had previously preferred. But you'll get used to it.
Also, remember that if your rotation feels too simplistic, artifact weapons in Legion will further complicate things, and they all come with an activated ability that will be a major part of your rotation.