You got to take into account the printer issues, releasing in the spring allow the MM & Vecna adventure in the Summer and the DMG in the Fall, with the the possible RD&D campaign book for FR in winter.
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...?).