Overwatch - a first-person shooter - also marks the first time the studio has launched a title worldwide on PC and consoles simultaneously.
WoW is the second bestselling PC game of all time, but its subscriber numbers have halved since their peak in 2010.
Blizzard predicts strong sales of Overwatch and hopes it will attract large audiences as an e-sport.
The game is a departure from the developer's previous hits, in which players usually viewed the world from a god-like, third-person perspective.
Instead, players look directly down the barrel of their weapon as they roam a colourful world battling other gamers.
"Blizzard has a huge following on a global basis and is one of the best renowned publishers," said Piers Harding-Rolls, head of games at the IHS Technology consultancy.
"But it has no pedigree in first-person shooters (FPS), so people will be excited to see what potential the game holds."
The game is also Blizzard's first new "intellectual property" in 18 years, since the studio has focused on spin-offs and expansions for existing franchises such as WoW, Diablo and Starcraft.
Subscribers to WoW have dropped from a high of 12 million in 2010 to 5.5 million in 2015, and the company has since stopped reporting the figures.
Thomas Tippl, chief operating officer at Activision Blizzard, said Overwatch would be "Blizzard's biggest launch since Diablo 3 in 2012" and hoped it would "resonate well in e-sports".
"E-sports is not only a driver of audience expansion, but also importantly drives growth in terms of time spent with our franchises," he said during an earnings call on 5 May.
If it captures the attention of e-sports competitors, it could have a very profitable future.
"Professional gaming competitions were watched online by roughly 200 million people in the past year, whilst over $200m (Â£138m) has been awarded in e-sports tournaments to date," said Dr Mark Johnson, a games researcher at the University of York.
"In recent years, Counterstrike has been the only highly successful FPS game with substantial longevity for professional gaming, so a game like Overwatch certainly has the potential to grab a huge new slice of this market.
"However, many FPS games have unsuccessfully tried to carve out a long-term sustainable place in e-sports, so it remains to be seen whether or not Overwatch has the right formula to knock Counterstrike off the pedestal."
Overwatch has already piqued the interest of many gamers, with almost 10 million players taking part in an early road test of the game.
Officially the trial period was to "test servers" and iron out any problems, but the studio's chief financial officer Dennis Durkin has acknowledged: "We're doing a tonne to expose players to the richness of this world."
"We learned a lot from the open test," Michael Chu, senior game designer at Blizzard, told the BBC. "We value the input of the community and we have been changing things here and there, addressing player concerns."
One of those concerns was about the portrayal of female characters. Some players complained that an over-the-shoulder victory pose for Tracer drew particular attention to her buttocks, while others questioned magazines featuring female characters littering the floor of an in-game toilet.
"There was a victory pose that we felt like we could make better and more exemplified the character. So we changed that," said Mr Chu.
The magazines were removed too, and the company says it has worked hard to create a diverse cast of characters.
"One thing we hope you'll notice in the game is that we have this big roster of heroes. We have some solider-like characters who have a military style, but we also have characters like Mercy, she's a guardian angel who moves around the battlefield healing.
"We wanted to get players working together as a team to achieve victory."
Despite speculation that Overwatch might be free to download, with revenue generated from in-game purchases, Blizzard said it had always intended to sell the game as an upfront purchase.
"It was always going to be a boxed product," said Mr Chu. "We feel like that is a really good way to go because so much of the gameplay comes from switching between heroes during combat, so having all the characters available rather than locked away was really important."
Unlike World of Warcraft, there will be no subscription fee to play Overwatch online. However, there will be in-game purchases in the form of Loot Boxes that can be used to customise characters.
Blizzard says that these can also be earned for free and it has promised "new maps and characters" will be added to the game at no cost.
World-of-Warcraft-maker Blizzard has released its first original gaming franchise in nearly two decades.