Diablo IV?

Technically per Chris Metzen, this D3 (plus its two expansions) is the last installment based on this particular era in this specific part of the storyline.  It does not mean the end of Diablo though as far as the ongoing saga goes.

You say this is the end of the Diablo “trilogy”? Does that mean we can’t expect more games set this in this universe from you guys?

Not at all. I see this as the ending of a specific storyline. It’s not at all the end of Diablo, the end of a particular age or era, but it is the end of a series of machinations that have been playing out on Sanctuary for the last couple centuries. If Diablo ever really had a plan to mess people up, to bring about doom and destruction, this game really illustrates how grand that design has always been. Before, we didn’t have all the information. This game shows just how smart and cunning he’s been over the arc of time and why he’s the series’ unique villain. In the first couple games there wasn’t a whole lot more detail to that character other than that he was a big red dude who kicked the s— out of people. So why is the series named after him and not Baal or Mephisto? Is Diablo stronger? I would argue that he’s just really clever, he thinks laterally in a way that his brothers do not. He’s not just one of the seven big Evils. He’s Doctor Doom. And Diablo III is the culmination of his machinations, even if it’s not the end to potential stories we could tell in that universe.

Source: http://www.ew.com/article/2012/05/17/diablo-3-chris-metzen-interview

The basic idea behind the expansions are likely already worked out.  The actual story board is one of those things that is largely set in stone where the two expansions are likely going to flesh out the actual details as to why the events that occurred in Diablo III, took place the way it did).  It is possible some of that content was also done along with the regular game.  The devil is usually in the details based on the changes/balancing that took place along the way in the vanilla version.  Plus no one knows if some of the content (like Paragon for example) were initially ideas that were going to be implemented in the expansion, which were brought forward.

The thing is the game has issues which this development team did not anticipate would happen (even though it is clear that they received feedback back in 2010 that the direction and decisions regarding some of the systems they went ahead with, would result in what we’re seeing now).  Expansions are budgeted based on the premise that it will be built mostly on top of the core systems that are in place on the vanilla game.  Thus the D3 team has to resolve those issues, while also fixing class specific ones (as they’d mentioned before regarding both the monk and wizard classes where some of the changes they want to do goes beyond simple numbers tuning), and also implementing a proper PvP system, all before they can get to actually begin marketing the first expansion.  And because their iterative design process isn’t the fastest methodology around, it’s going to take them some time to properly address these fundamental issues first.

Whether or not this actually translates into a Diablo 4 in the future will really depend on how this development team resolves the known issues that are plaguing this version of the game.  Why?  Because too much of the goodwill legacy that fueled the sales for this version of the game, were likely a result of the first two games in the series.  Some of that goodwill has now been damaged by this particular game where only a fraction of those who purchased Diablo III, will be automatic sales for either expansions.  The way they actually deal with itemization in whatever patch that takes place in, will be key to the overall future health of this franchise.

I’m not saying the sales won’t be successful; it likely will do alright but just not in the staggeringly large numbers it could be had this game been handled a lot better.  Only Blizzard themselves know the real numbers though based on player concurrency and actual time spent in-game though.  Considering how actual officials are stating the game has issues, it is possible they don’t like the trend of the concurrency numbers they are seeing for this game.

I fall into that particular camp as far as one who played both the original Diablo, Diablo 2 + Lord of Destruction.  I own more than one D3 license key (one of which is the Collectors Edition) for stash purposes. I also play SC2 and got the Deluxe version of Heart of the Swarm.  The point is that Blizzard knows my marketing preferences and the demographic I’m in.  The D3 development teams handling of this franchise however, translates into someone who isn’t going to be in the market for any of the D3 expansions if they do not properly address and fix the issues in this plain vanilla version of the game.  And it is clear I’m not the only one who feels this way; which as far as this franchise goes, is not what you want to hear from your customer base.