I'm particularly excited about how they'll implement the upper house/legislative branch in Vicky (I've never played Vae Victis) and the spheres of influence system.
There's been some theorising about how the spheres of influence system could work on the Paradox forum. One idea is that it'll be kind of like the Axis/Allies/Comintern system of HOI3, except with the Great Powers instead of the Axis, Allies and Comintern (and not as connected to alliances). Of course, that brings the problem how they'll handle changes in Great Power status, Great Powers influencing each other...
Interestingly, their new EU3 expansion will
also add a spheres of influence system. That could be one reason why Vicky2 will come out so soon, relatively speaking- they can take H3T's SoI system, and modify it, just as they've taken HOI3's map, and modified it. Or the systems could be different, or it will be/is H3T's team that will take Vicky2's SoI system and modify it....