Dragons!
What we know so far:
Our 3rd edition update comes with tons of new models and warscrolls
Old warscrolls and allegiance abilities are getting revamped
Loads more battleline choices
Hammers of Sigmar gets battleline Dracothian Guard
Sequitors battleline without a Lord-Arcanum general Vanquishers are battleline
Knights Excelsior get battleline "Paladins"
A Knight-Draconis general makes Stormdrake Guard battleline