...or whether to unite disparate provinces into regions – another new mechanic.
The what now?

Regions (the single areas) have been able to unite into provinces for a while in the base game (maybe since release, can't recall, but a long time). I've been playing the new mechanics for the base game since Persia released (finishing out a run as Judea, also with some nice scripted events btw), and have not seen anything yet about uniting provinces into larger groups.
What IS new, are "region decisions", which can be applied to valid regions (not to provinces) for special effects, at various costs. So far the handiest is the ability to spread slaves around to adjacent territories, since the capital tends to pick up a lot of slaves from conquests which tanks their ability to do much of anything and incites revolt risks -- as you might expect.