The Hearthstone Spring Championship Tournament is the only upcoming big Hearthstone event, it is on July 7-9 weekend held in China. Considering all the promotions along with the event, it will most likely be the perfect time to announce the next expansion. There might be early invitations, secret handlers, event teasers, etc in early July. After that announcement, they will start revealing new cards, mechanics, etc throughout July. I expect the expansion to hit early August.