Called Season of the Malignant, the new season will include a 6 new Unique items, 7 new Legendary Aspects, and a whole host of monsters to defeat. The new Season will also encourage players to finish the game’s campaign: according to announcement (which you can read right here), “To participate in Seasons, a new character must be made on the Seasonal Realm. You do not need to purchase a Battle Pass or any add-ons to play the Seasonal content, but you must have completed the campaign on either the Eternal or Seasonal Realm with at least one character.”
Some content will carry over, but based on what Blizzard has said, this Season — and others to come — will be their staging ground for new ideas and new content. They’re promising more details ahead of the July 20th launch, so stay tuned!