I found very intriguing that Diablo Immortal was not mentioned during the BlizzCon 2019 Day 1 panels. Instead, Blizzard Entertainment posted a Diablo Immortal update article in the Diablo III website.
They provided a short FAQ. BlizzCon 2019 attendees can play a Diablo Immortal demo at the showfloor where they can test the Barbarian, Wizard, and Monk (same as last year), but now with the addition of the Demon Hunter. Attendees can play through the town of Wortham and the outdoor zone of Ashwold Cemetary.
Blizzad Entertainment added a new base game mechanic: Ultimate Abilities — “All classes gain charge for their chosen Ultimate by using basic attacks. Once this charge has been built up, players can choose to augment their basic attack for a short time in a face-meltingly powerful display of strength, guile, or raw magical potential.”