Now that I have the book (picked it up from one of our local Premier stores), I can say that I'm seeing a great potential analogy to the potential usefulness of the SCAG in the contents of page 5 in the book's Preface.
Page 5 is a wonderful-looking large-scale map, stretching from the Spine of the World in the north all the way to the Forest of Tethir and the Nelanther Isles in the south, and stretching as far east as Cormyr. You can tell at a glance that most folks travelling from Waterdeep to Mirabar are doing so overland (and how), while most folks going from Waterdeep to Baldur's Gate are likely going by ship (and why). For someone playing in Encounters, or in a campaign spun off from one of the hard-cover Encounters-legal adventures, it's a great resource all by itself.
And for folks who almost exclusively play Expeditions, it's pretty but useless -- you'd need another map of similar size and scale to even get as far east as the Moonsea and the cities on it that comprise the adventure areas referred to in Expeditions modules.
If there is an announcement in the next few days, I suspect it will be to clarify the distinction between setting material (which comprises the first two chapters and roughly the first 100 pages of the book) and player options (which start with Chapter 3: Races of the Realms and covers the race, class, and background options that compromise the last roughly 50 pages of the book). It doesn't make a whole lot of sense, for example, to say that a new character created as a Season 4 character would be eligible to worship Deneir, Lliira, or the Red Knight (all, I believe, deities that don't appear in the Player's Handbook), but that a new character created as a Season 2 character would not be eligible to worship those deities (assuming SCAG is not made legal for seasons prior to Season 4).
A few other bits of note from an initial perusal of the book over lunch, in keeping with the speculative intent of this thread:
- Genasi are barely (if at all) mentioned in the book; they have no index entry and do not appear among the races discussed in Chapter 3. As such, genasi in AL are still only legal as Season 2 (Elemental Evil) characters. Unless SCAG player options are made legal for Season 2 characters, you will not be able to make a genasi eldritch knight (swordmage) with the Greenflame Blade cantrip.
- There are a number of places in the SCAG that refer back to the Player's Handbook: as an example, dragonborn, rock gnomes, and forest gnomes all refer to the PH when talking about racial traits -- no new traits are presented in SCAG for those races.
- Svirfneblin, however, refer back to the PH, and not to the Elemental Evil Player's Companion. The EEPG combines the PH and svirfneblin traits in its entry, while the SCAG only lists the svirfneblin traits different from the generic gnome traits in the PH, so while they appear to be the same to me, any difference in those traits will likely result in a difference between season 2 deep gnomes and season 4 deep gnomes.
- Tieflings refer back to the PH, with the exception of tieflings that can trace their ancestry directly to Asmodeus, where those abilities are called out in a sidebar. It is almost certain that the Winged ability in the sidebar will not be allowed in AL (for the same reason aarakocra from the EEPG were banned), and possible that the entire sidebar will be disallowed for AL play, since it requires a connection to an evil deity that might not be seen as 'kosher' for public play.
- People who have been asking for a 5E warlord: meet the Purple Dragon Knight fighter archetype.
- The paladin section adds a bunch of traits that all paladins in the Realms are expected to uphold, regardless of their specific Oath. A number of these 'virtues' are going to be challenging for players who've built an evil paladin and are playing him in 'evil-jerk' mode, though there's no specific penalty for failing to uphold the traits, and the text does specify that different deities favor different traits and gives a couple of examples. I predict that this is going to cause more arguments in AL than perhaps any other part of Chapter 4.
- The Warlock section gives explicit examples of the different warlock patrons that exist in the Realms; I predict that, if SCAG's player options are 'back-ported' to seasons prior to Season 4, that choice of patron will be required for warlocks just as choice of deity is for clerics.
- The SCAG notes that the racial restrictions on battlerager barbarians and bladesinger wizards are for the Forgotten Realms and might not apply to a given DM's setting or version of the Realms; however, since AL is presumed to be running on a 'baseline' Forgotten Realms setting, expect these restrictions to be maintained in Adventurer's League.
Just my $0.02US -- I expect that at least one of these predictions to be proven wrong by the actual announcement, coming Soon(tm).
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Pauper