The question of brand new armies vs updated classics is an interesting one. There's some obvious examples of brand new armies, like Ironjawz, Deepkin, Fyreslayers, ect.
And some obvious classic armies like Skaven, Beasts of Chaos, or Seraphon.
OTOH, there's some in-between options that I'm not sure how GW classifies them. You have for example FEC and Beastclaw Raiders, new army concepts using entirely old sculpts. You also have releases like Daughters of Khaine or Gloomspite Gits, building old concepts into heavily revamped armies with a lot of new models.
Would Darkoath or Slaannesh Mortals be "brand new armies" in GW's eyes? I honestly don't know.
That said, if we go faction by faction looking for possible "new" armies:
Chaos: With Slaannesh and Darkoath all but confirmed for this year, along with Skaven and Blades of Khorne already getting updates, I would be very surprised to see any other "new" Chaos armies in what is already shaping up to be a very Chaos heavy year.
Destruction: Gutbusters I think is looking to be the next big Destruction release with how it was previewed as a combined Gutbusters/Fire-eaters/Man-eaters army in the leadup to 2nd edition, and the way it's organized as such on the store, and described that way on the faction page. How many new models they get is the big question but I do feel like Ogres are due to get a big update to bring them up to par with Gloomspite/IronJawz and establish them as the third major tentpole faction for destruction.
There's also mention of Grotbag Scuttlers going back at least as far as the KO battletome, but I don't think we'll get another new Goblin army so soon after Gloomspite, and there's been no rumors of them coming that I'm aware of.
Death:
Death is pretty well wrapped up currently, with Nagash, FEC, and Nighthaunt covering the entire current range, so any new armies they get will be fairly out of left field. I could see building Soulblight into a new army maybe, but again I don't know of any real rumors pointing in that direction currently.
Order:
As mentioned, the new Light Aelf and Dark Aelf dynamic is very well established and are inevitably coming at some point. Are one of them coming this year? Maybe, and they do seem like the cleanest fit for "brand new faction" we know about currently, but it's hard to say if they can be squeezed in this year or not.
Otherwise, Order is the faction most in need of some consolidated army-books to clean up the Human/Duardin/Aelf micro-factions but I don't think any books along that line would count as "brand new" armies.