Henadic Theologian
Legend
OK, so detailed speculation breakdown, let's go.
We might see a Setting product, either Greyhawk or Forgotten Realms, but nothing is super determinate for that right now.
We know that we are getting a big flashy Campaign starring Vecna, most likely in late September at the close of Q3. Most likely made with the idea that people can use either set of Core books.
We know we are getting three core books, staggered over the year instead of released as a batch.
Since 1977, the PHBs have dropped between April and August, with D&D'14 dropping day and date with Hoard of the Dragon Queen. So a Q2 or Q3 release is likely based on historical precedent. Releasing the PHB before Vecna is fine, since there is a decade worth of potential tie-in products.
We may see the MM drop in Q1 or Q2 before the PHB, or in Q3 or Q4 agyerthe PHB. History shows that can go either way, and just using the new Monsters with old options would be fine, so there is no necessity to the release order.
I suspect the DMG will be November or December, as a Christmas release, but we may also see another Setting product in that area (Greyhawk slipcase, Forgotten Realms box set...?).
I think the DMG gets Greyhawk, a backdoor way for Chris Perkins and crew to get the setting for the anniversary (and maybe released to dmsguild) and FR gets either a Campaign Book updating the setting since SCAG or a Slipcase like Planescape.
I'll be surprised if the core books aren't released in order of playtesting.
Also remember playtesting for the MM and DMG will be alot less then the PHB, I expect 2 or 3 playtests at most for each, I think the DMG will be released before winter, Spring, Summer (along with Vecna, but different Month), Fall, setting book Winter/Christmas.