Tesla's mission statement is to accelerate the world’s transition to sustainable energy. During Tesla's Q1 2020 conference call, Elon stated that battery day will be "one of the most exciting days in Tesla's history".
Actually, we don’t want to preempt Battery Day. We want to leave the exciting news for that day, but there will be a lot of exciting news to tell. And I think it would be one of the most exciting days in Tesla’s history and we’re just trying to figure out the right timing for that.
I'm not sure what specific tech will be announced; lots of clues and puzzle pieces out there. However, I do think the most important thing Tesla can do to accelerate its mission is to make EVs so much more compelling than ICE that it's a no brainer (i.e. a hardcore smackdown). I think this will ultimately come down to price.
My prediction: On battery day, Tesla will announce new battery tech and production that will allow Tesla to bring the cost of its EVs below ICE vehicles.