Blizzard is clearly focusing on selling Overwatch as a fixed product, and hasn't begun to think about additional "DLC" heroes.
Blizzard somewhat surprised us at BlizzCon this year by revealing that it's upcoming shooter Overwatch will in fact be sold as a single purchase boxed product, rather than being free-to-play with Heroes of The Storm-style microtransactions as was predicted. However, people are growing concerned that when (or rather, if) new heroes past the 21 launch heroes are added, we'll have to open our wallets again. Well you can rest a little easy, as Game Director Jeff Kaplan has told PC gamer that Overwatch is clearly not going to adopt a Heroes of The Storm payment model.
Responding to accusations that Blizzard has been "dodging" the question of post-launch content, Kaplan stated: "That's very upsetting to me, because dodging to me would be, 'We have a definitive answer and a definitive plan and we just don't want to tell you what it is yet,' and that's absolutely not the case. We're not sure if and when and how we're going to add new heroes to the game at all."
"I think there was a misconception that went around early on-and I'm not sure how this came about-that you would get 21 heroes when the game launched, but there would be a 'hero store' with other heroes for sale at launch also, and that just couldn't be further from the truth," he continued. "There was also a misconception that we would be selling maps, and we've never had any intention of selling maps."
So the definitve answer from Blizzard is "we're not sure", but we at least know that new maps will never be paid downloads, which is good because we've seen how paid DLC maps in online shooters like Call of Duty and Battlefield can really split the community.
My prediction is that all new heroes will be released for free, and Blizzard will simply recoup their costs by selling cosmetic skins, a la DoTA 2. However, there is still some cause for concern, as Blizzard is after all the company that brought us $10 animated .jpegs.
Source: PC Gamer