The Blizzard website's starting page has been changed to a splash image showing each game they've released in the past. There's also a timeline at the bottom which lists each of the games, including a question mark for the game being announced at the Worldwide Invitationals less than a week away.
Each day the splash page is going to be changed, until the big day that the new game is announced on the 19th of May. Make sure to keep an eye on the website for other possible hints!