Original announcement VOD:
https://youtu.be/WZxTihBkkpQ?t=789 (starts at 13:09)
I think I'm most exited for Zendikar: Rising. Being a D&D nerd as well as MTG I love the adventuring party theme of the original Zendikar and would love to see the plane again without the Eldrazi being the main focus for the entire set.
Ikoria sounds pretty interesting. It could be linked to Theros in some way: it's the plane of monsters, and Theros is known for it's monsters.
It'll be interesting to see how Theros deals with bringing people back from the dead. I'm not a huge fan of
resurrection in stories in general, as it takes away a lot of the impact a character death can have as the question is always "why don't you bring them back?" But, if they handle it well, it could make for some interesting lore and could allow some good story arcs to happen.