Diablo turns 30 this year, and Blizzard says fans will freak out at BlizzCon
The tease is real, so what could Blizzard plan to reveal during the opening show?

This year, we are celebrating 30 years of the Diablo franchise at BlizzCon. There is a lot of speculation in regard to what will be shown for Diablo 4 during the opening ceremony.
At the end of February's 30th anniversary spotlight, Diablo IV's Game Director Brent Gibson looked away from the camera and said, "Are we good? Are we clear? Oh my God, they're going to freak out at BlizzCon when we announce…" Then the video cut. (You can see the respective timestamp here, or check out the full video below).
It was a very deliberate choice from Blizzard. It did not tell us anything, except that we should wait for BlizzCon for exciting news. We know they deliberately hold back some announcements for their big opening show, and we know Gibson expects us all to be shocked.
This has triggered many theories and discussions within the community. It will very likely be related to Diablo 4, as we already know Blizzard has more Diablo IV content planned for the next few years.
What We Think Is Coming
We learned with the Season 15 PTR launch that the content of the PTR was not the entirety of what players should expect for the next season. Rebirth and Legacy Uniques were available for testing, but they should only be a small part of the seasonal package that will start a few days after BlizzCon.
We could be looking at a new endgame system, a permanent feature, an anniversary event, or another seasonal mechanic that Blizzard wanted to reveal on stage. But this would be quite normal.
A new roadmap preview for the upcoming months would also make a lot of sense.
Or Blizzard could easily tease the next chapter in the story without announcing a full expansion yet.
We think teasing a new piece of the upcoming story is the closest prediction. We do not believe Blizzard would call out that players will freak out if it were truly just about additional Season 15 news. Gibson's reaction makes it sound like there is another, bigger surprise than the usual season preview.
So, what could Blizzard announce at BlizzCon that would make you "freak out"?

