Taken on its own and at face value it's hard to gauge the degree of impact that Blizzard's mysterious April 11 reveal will have on Overwatch. However, Renaud Galand, who is the lead character artist on the game took to Facebook to express his excitement about this announcement by saying it's essentially going to be the best thing ever:

You know the hype is real when people who have been working on the game call it the best thing since launch, especially when you take into account that thus far Overwatch has seen its fair share of events and new character releases, not to mention that the one year anniversary of the game is coming up in May.

Additionally, shortly after Galand's Facebook post, Jeff Kaplan, game director of Overwatch, hinted during an interview with PCGamesN that the big reveal will in fact be a playable event but it's also more than that since unlike past events it won't be based on a real-world festivity, instead much like the web comics and animation shorts it will be aimed at players that want to know more about the in-game lore of Overwatch and it will serve to expose the past of Overwatch as an organization and how things got to the state where they are today. Kaplan explained it as follows:

All this considered, one can only be led to believe that this might end up being a substantial announcement or at the very least one that may offer players a new type of experience compared to what past events had to offer. 

Sources: VG24/7 and PCGamesN