My suspicion is that they're churning out the sub-faction books first because they need the fewest new/re-boxed sets. Spend less time/money on the models and more on the books to crank them out faster.
That said, I would be very surprised if we didn't get at least some Xenos releases in the initial 10 books. Tau seem pretty likely since they have very minimal old metal/resin sets (are there even any left in the current range?), and even their first-generation plastic sets (Kroot, Fire Warriors, Hammerhead/Devilfish) are aging pretty well. That said, given how little recent storyline events have focused on the Tau they might get backburnered for more central/iconic races.
Eldar played a pretty pivotal role in the recent storyline, and even their old kits (e.g. Vyper, Falcon, Guardians) are holding up pretty well. Aspect Warriors are a problem though, since they're still the old metal/resin casts. Warp Spiders are sorely in need of an update (the "current" models date back to 3rd edition). Same goes for Swooping Hawks. Shining Spears could be done by just adding an accessory sprue to the current jetbikes. But Banshees, Scorpions, Dragons, and Reapers, despite getting new metal/resin kits a while ago, are probably due for a re-re-release in plastic to get the whole line up to date. Also, some plastic Storm Guardian arms might be nice. Added up, that's a lot of new kits that need to be made, and that doesn't feel like a "before 2018 along with 9 other books" project to me. I'd love to be wrong, but Eldar I can't see coming out properly for a while.
Tyranids I could see coming out soon-ish though. They've got some nice centrepiece plastic sets, relatively little remaining metal/resin, and they can lean on the new Genestealer sets if they needed to. Plus Tyranids haven't gotten a new book in a while, so that'd be nice. That said, because they've been neglected for so long I could see GW releasing them as a flashier Eldar-style release as I described above, with lots of new sets.
IG seem like another line that they could churn out quickly. The Cadian line has been pretty stable for a while, and making some bundles with lots of tanks isn't hard. Maybe make some new accessory sets or character models, but I can't see much that's urgently missing from them. It'd be super nice to see some of the old regiments make a return in plastic; I have a soft spot for Tallarn personally (I own about 7kg of the old metal ones), but that's probably further off. Maybe save that sort of thing for IG sub-faction releases? Codex: Steel Legion, Codex: Catachans (again), Codex: Vostroyan Firstborn, etc...
Skitarii/Ad-Mech are probably another quick turn-around army. Their models are all pretty recent, there aren't any old metal/resin kits that want converting to plastic.
And then we're back to the SM and Chaos sub-factions. Deathwatch are likely an easy one, since they can leverage every other chapter-specific release for conversion parts, and the Kill Team boxes (Cassius + the normal one) are recent, good-quality sets. Blood Angels have lots of nice plastics already (e.g. Death Company), and they're almost as iconic as the Ultramarines in a lot of the production artwork. I'd love to see a Codex: Renegades to cover IG turned traitor, along with a proper Daemons book that eschews corrupted humans entirely, but I can't see either of those happening in the initial 10 books.
This post has turned out more rambly than intended, so I'll perhaps sign off here and let everyone get back to the discussion.