The rumor comes from someone alleging to be an intern at Blizzard and is part of the Diablo 3 team. Apparently Blizzard will only debut a simple trailer rather than a full-blown CG trailer at BlizzCon 2015 so perhaps this might not necessarily be a full-fledged expansion, but rather a DLC of sorts. The DLC is also rumored to introduce a Rune Words system to the game that will apparently be very different from that of Diablo 2.
It will be interesting to see if this rumor pans out, especially given the fact that Blizzard has only ever released one expansion per Diablo game, not to mention the first Diablo expansion was not even made by Blizzard to begin with. Blizzard has introduced new patches that add new features and items, but new content would be a different story.
Take it with a grain of salt for now, but considering that the overall game still feels a bit grindy despite the addition of new events, we’re sure there are some players out there who would welcome new content and stories.