The announcement of Overwatch, Blizzard's first new IP in 17 years, confirms for me what Blizzard's design philosophy is. Blizzard does not create new genres anymore, it isn't their thing, and that's actually perfectly acceptable.
What Blizzard does now is take an existing genre and release a polished, accessible version targeted towards the middle of the casual-hardcore Gaussian. This approach popularizes their games and opens up gaming to wider audiences which I think we can agree is a good thing overall. After all, how many gamers might never have picked up a mouse and keyboard if not for a friend introducing them to WoW?
Get the pitchforks and torches ready...
Now before anyone comes for my head, ask yourself what genres Blizzard actually invented. The truth is they arguably only invented one, and even that is up for debate.
RTS? Actually creation of the top-down, base building RTS genre as we know it is credited to Dune 2, created by Westwood.
Online CCG? Magic Online was released in 2002. Hex, Scrolls, and several other smaller games also beat Hearthstone out the door.
MOBA? Get out.*
MMORPGs? No seriously, get out. Sarcasm aside, while the crown is debated, WoW was nowhere near the first. It does however provide an example of the "polish + access" philosophy that has become Blizzard's mainstay. WoW is easy to approach, friendly to many skill levels, visually appealing (and not overly violent), and polished (at least compared to other MMOs). A player did not need MMO experience to pick up and enjoy WoW and it didn't require much out-of-game research to succeed in.
The only genre Blizzard can really be credited with is the point-and-click action roleplaying game, Diablo. However, it's future incarnations did less genre-definition and focused more on...wait for it....here it comes...you guessed it! Accessibility and polish. It's most recent, Diablo 3, has arguably made no innovations on the genre, but has released a well executed product that is easy for a multitude of players to enjoy.
None of this is necessarily a bad thing, if nothing else Blizzard's financial success has shown that the market agrees. Other companies should look at Blizzard's example of just how polished/accessible products should be if they want to appeal to massive audiences. Blizzard may not be the inventors, but they are particularly good at spotting the rough edges of a genre and sanding them down smooth. In a lot of ways Blizzard has become the Apple of video games; we can quibble about technical aspects of their products, but we can't deny that they are incredibly easy to use.
*While DotA, arguably the first MOBA, was created in Starcraft's engine it was not created by Blizzard. The MOBA belongs to Blizzard no more than the Mona Lisa belongs to the inventor of paint.