ESO+ subscription will get you everything except for the Greymoor Chapter for $15 a month.
Otherwise, the Imperial Edition only includes the base game. You'll need to purchase any DLC you want as well as the new Chapter Greymoor. With the preorder period for Greymoor over, I believe the digital upgrades for Greymoor only include itself.
What I'm not sure about is whether you can buy a Greymoor standard edition (which includes the other chapters as well as the base game) and apply it to an existing edition of the game.
so I have renewed my eso plus I just need to buy greymoore. Thanks for the info.
I forgot about the new classes. Sorry, the Chapter to DLC conversion is confusing. ESO+ gets you all the DLC, but that doesn't include the classes that were part of previous chapters.
If you want Warden or Necromnacer, they are purchased separately through the Crown Store, now. I'm sorry if I misled you there.