Well see here's my theory on why a D4 is probable, and a smart move by Blizzard.
I don't even see the point of D2 HD. I don't want to play a game where I have a crapload of potions in my inventory. I don't want a game without a shared stash. I don't want to gear up by doing endless Baal runs. D2 is a dinosaur by modern gaming standards. If they update the graphics without updating the game play, it will be a novelty. If they update the gameplay, then it's a whole new game. (And if they were successful with such a gameplay update, they'd just be cannibalizing the Diablo 3 player base, which is currently their marque Diablo game.)
The longer D3 goes without another expansion pack, the more I believe they are working on D4. But I can't figure what they'd do in D4, that they could just do in a D3 expansion. Moving to D4 would allow them to a) eliminate all the legacy of people with thousands of non-season paragon levels, b) allow them to reintroduce characters like necromancer without stepping on the novelty of existing characters, and c) ???. I'm drawing a blank here.
I'm just not certain what would be "different" enough to warrant a whole new game. Sure, new classes and new story and new items. But those could be done in an expansion, right? Would the gameplay be different? Could they make it truly open world rather than 4-man (or 8-man) instances?
Many of the complaints about D3 stem from the systems they've introduced. And these systems were themselves bandaids for other complaints, and it just piled on. I'm talking about the Paragon system, leaderboards and the Greater Rift system mostly. For example, the Grift system as well as leaderboards completely breaks a large part of the game - builds and playstyles. Balance of all 24 sets + LoN builds is nearly impossible when you've got an infinitely harder system that works against it. No matter how you balance the game, Grifts will ALWAYS shave away any build that isn't in the top percent. And that's just one system.
The problem however is completely reworking these systems, or getting rid of them would be far too much a shock to your average player. Most of the players don't spend time here or on other forums, console players included. I think Blizzard is scared (and maybe rightfully so) that changing too much in the form of another expansion for D3 would cause too much chaos. Changing things such as removing the power gained from Paragon points would be too much change.
A smarter move would be a whole new game. It's the perfect excuse. Starting fresh means they can add the classes people ACTUALLY wanted to play, they can move the game BACK to a loot - focused game instead of Greater rifts and away from an XP grind, ect.
So while I'd welcome anything new, I'm *really* hoping for a D4.