We are about due for a Diablo 3 expansion since the last expansion Blizzard released was Reaper of Souls which was released back in 2014. Rumor on the street is that Blizzard is getting ready to announce new content for Diablo 3 at BlizzCon 2015. The event will be taking place toward the end of the year, so we will have to wait and see.
The rumor comes from a source that is allegedly an intern at Blizzard and is part of the Diablo 3 team. If true, then they would know what they are talking about. But take it with a grain of salt for now until we hear something official.
Apparently Blizzard will only debut a simple trailer and not a full CG trailer at BlizzCon 2015, so we wonder if this is an expansion, or just a DLC. The DLC is rumored to introduce a Rune Words system to the game that will be very different from that of Diablo 2, but again we have limited info at this point.
Blizzard has only ever released one expansion per Diablo game, so we will see. We know that players out there will welcome new content and stories if Blizzard is inclined to deliver them to gamers.