Blizzard announced that large-scale gathering is going to be problematic in 2021 so BlizzCon will be delayed and take a different form.
BlizzConline wasn't really a bad experience, especially because many got to check it out without the paywall. However, it was born out of necessity due to restrictions imposed by the Coronavirus pandemic and it seems like the story will mostly be the same with the next convention.
The company announced that the complexity and uncertainty of the pandemic caused the organisers to be unable to create an environment where the participants could have fun and health security at the same time.
While this might sound like incompetence on Blizzard's side, it's only fair to consider what it would take to make sure a BlizzCon doesn't become a hub for spreading the virus. Additional measures would include making sure no one entering the tightly-packed space is infected and since people become contagious two days before the symptoms even occur , you can see how this could become an impossible mission for the organisers.
That said, the BlizzCon is not cancelled and Blizzard folks are working on getting people in for limited live events. It will be moved from late 2021 to early 2022 and will mostly consist of an online convention.
Furthermore, Blizzard will attempt to combine the online show with smaller in-person gatherings but more will be revealed as their plans develop.