Looks like Zariel is on par with the most powerful of the demon lords (going by CR anyway). I'm looking forward to seeing what the other archdevils are like, I'm assuming that they are all around the CR 24-26 range like the demon lords.
So the question is, which ruling Archdevils are we going to get full stat blocks for? Zariel is the first ruler of a layer that we've seen confirmed - all the other Archdevils revealed so far have been either sub-commanders or deposed rulers. We know we won't be seeing Belial or Baalzebul since we've seen the Roll 20 preview of the creatures that begin with "B" in the book (regardless if they will actually be in the "B" section), and they weren't listed there.
Right now I'm guessing:
1.Zariel - confirmed in.
2 Dispater - unknown. (My opinion: not in. They seem to be focusing on those adventurers are most likely to encounter, and Dispater is notorious for rarely leaving his palace in Dis for defensive reasons).
3 Mammon - unknown. (My opinion: in. As the patron of greed, he could definitely be encountered to tempt adventurers to sell their souls for wealth).
4a Belial - confirmed not in.
4b Fierna - unknown (My opinion: maybe. More likely of Glasya doesn't make it in, as they are relatively similar).
5. Levistus - unknown (My opinion: not in. He's stuck in the center of an iceberg, so no one is reasonably going to encounter him).
6 Glasya - unknown (My opinion: in. Being the daughter of Asmodeus makes her an interesting character to profile).
7 Baalzebul - confirmed not in.
8 Mephistopheles - unknown (My opinion: in. Well known, and notorious for interfering in the affairs of mortals).
9 Asmodeus - unknown (My opinion: not in. They'll wait for some sort of book that covers deities before giving him stats, if even then).