It is high time we say what needs to be said—Diablo 3 does need new content. With how it was designed to be, which is an action role-playing game with an always-online experience, then it should be a given that Diablo 3 is to get a lot more regular support than its predecessor Diablo 2 ever did. Blizzard does not need to come up with anything as big as Reaper of Souls, but at least a new area or boss every now and then would be nice.
With how Diablo 3 ended up being, not the critically-acclaimed hit that Diablo 2 was in the long term (it got rave reviews in the beginning before its cracks really started to show), but now in a niche that it has gotten itself comfortably in. There are plenty of players out there who play this game right now and think it is indeed a right good time playing it, but it is starting to get that "same old, same old" feel now that its last major content update being quite a while ago. Therefore, it is time the game gets some freshening up.
Releasing something like a new expansion would invite some new players in and bring back old players who have stopped playing Diablo 3 for a long while. That is usually what happens whenever Blizzard releases a new expansion for any of their games, and Diablo 3 would not be any different. However, creating new content takes a lot of time and resources, and Blizzard happens to be a game development company that really likes to take its time with everything it makes.
Blizzard is also not well-known for making use of the DLC business model that other companies are known for. While they have done something similar recently with small expansion sets in Hearthstone and Starcraft 2, it is not known if they would ever do such a thing for something like Diablo 3 as well. They would most likely stick to the traditional expansion pack model like with Diablo 2 when they released Lord of Destruction all those years ago.
However, lack of new content over a long period of time means stagnation. They do have seasons now for Diablo 3, which means there is something bound to be exciting for each season as players vie for the top of leaderboards. However, this is only true for the more hardcore crowd in the game. As for the more casual fans who are only interested in seeing new Diablo 3 items and story while getting a bit of action role-playing game experience, then they would want to see something new every now and then.
There are also a good number of holes and cliffhangers that the Diablo 3 story still has, which should be worth filling in. That is definitely something fans would be interested to see right about now. Reaper of Souls did add a new dimension to this in-game world, but it may not be enough to have fans really sink their teeth into this world that served to fill the void that Diablo 2 left behind many years ago. That is certainly a major reason why Diablo 3 can use a new expansion now.