When Diablo 3 launched back way when there was not the level of competition on the playing field of action role playing games as there are today. Give a person choice, the ability to play prospector and suddenly our alliances to said faction can become a little foggy. The point is, many of the players who were shoe horned into D3 due to a lack of other options are now all over the place. Blizzard are going to have a much harder time filling orders for D4 than D3 simply because of the number of other arpg's out there.
So how can Blizzard make D4 the arpg that everyone wants in on? What does it need for you to pick it up for x$ over the many other arpg's out there that are cheaper or free and can introduce or improve on anything in D4 anyhow. What is the kicker that is needed to move D4 in your opinion?
Well, Diablo 4's style/visuals will play a part in enticing people. D3 got a lot of criticism for that. I also think game systems like the skill tree will be important for fans, especially D2 players who were disappointed with D3. Same goes for D4 Items and itemisation and how that will work.
Blizzard has lost a lot of goodwill over the past few years and they won't find it as easy this time to break through but the press will play a huge part in enticing players to pick it up. That is also a real danger because let's face it, the press is not exactly impartial when it comes to anything with a Blizzard badge on it.
Yes, a lot of people use youtuber's for news too. Those viewers may not account for a many as far as unit sales goes but many of them know people who talk. That whole word of mouth thing starts to kick in here. So if the youtube media don't buy D4, well. I mention them because they do tend to represent the more critical portion of the media. It won't matter how bad it is. There are some youtuber's that will also fall into this category unfortunately.
Blizzard still have to claw back the gamers who left for POE and Grim Dawn or just left for whatever reason. If they fail to do that then they will be dealing with a new market that could be stolen from them again. They are going to have to spend up to pull those guys in. Either way it all equals expenditure. I just can't see a way that D4 will not be expensive to play at least for the short to medium term post release to make up for all the expenses gone into it.