Mike Donais, a HS dev, said this in an interview yesterday:
Q: Can you spoil what you’ll be announcing at Blizzcon? We promise not to tell anyone.
A: Yeah, if you don't tell anyone. Blizzcon will be better than it has been in like 10 years. It is going to be amazing!
The question is asked at 01:28:41.
Now, let me clear some things since I follow HS a lot: HS at Blizzcon is always the same since release - new expansion announcement, which is done 3 times per year and one of these is at Blizzcon aka nothing special. This year won’t be any different since there are no major plans for changes in HS.
Also, mentioning “10 years” brings a thought what was HS at Blizzcon before 10 years - well, nothing, since HS was launched 5 years ago.
This means Mike is not referring to HS when he says “Blizzcon will be better than it has been in like 10 years”. Could he be referring to a potential big announcement like D4? I think this is highly possible.