My thoughts are this - Dark Sun is almost definitely the setting for next year. But this year is Dragonlance's anniversary so they will do something special for it - a One-off setting book at "double" length (400ish pages) It will be like the Campaign Guides in previous editions, with both player and DM stuff it in.
This goes against the 'one setting per year' rule, but it's a special year!
Instead of a setting, they may just be re-doing the Chronicles era adventures as a huge hardback too.
Who knows?
Maybe they'll do some one-off "double length" (400ish pages) books for some of the less popular settings (past saleswise), released at a rate of two or three per year?
After awhile, how many more "martial power X" books can and will they release? (Or for that matter: arcane, divine, primal, psionic, elemental, shadow, etc ... "power" books).
I imagine WotC will look back through their catalog of 3E/3.5E supplemental books, for other stuff to "strip mine" into new 4E books. Another source would be to steal some ideas for new books, from what some 4E 3pp publishers have released already.
Looking at the 3E/3.5E supplemental books catalog, stuff which possibly looks easy and/or viable to "strip mine" into possible new 4E books:
- faiths and pantheons, deities and demigods, etc ...
- epic level handbook, power of Faerun, etc ... (remade to beyond epic)
- book of vile darkness / book of exalted deeds
(champions of ruin / champions of valor)
- races (ie. races of stone, races of destiny, etc ...)
- multiple monster manuals, fiend folio, fiendish codex, etc ...
- libris mortis, lords of madness, drow of the underdark, etc ...
- more draconomicon books, dragon magic, etc ...
- unearthed arcana
- more manual of the planes, planar handbooks, etc ...
- environments: frostburn, sandstorm, stormwrack, etc ...
- evil (ie. exemplars of evil, elder evils, lords of darkness, etc ...)
- ghostwalk
- heroes of battle, heroes of horror
- stronghold builder's guide
- magic (ie. tome of magic, magic of incarnum, etc ...)
- historical (ie. lost empires of Faerun)
No idea if they will publish a new 4E series of "complete" books. Though perhaps the 4E "power" books are functionally serving the same purpose that the "complete" books did for 3.5E.
Towards the end of 4E's shelf life, I wouldn't be surprised to see a "Rules Compendium" book being released eventually. I don't think a 4E version of the "Magic Item Compendium" will be done, considering the Adventurer's Vault books are more or less serving the same purpose. No idea about a "Weapons of Legacy" book.
If persuaded, maybe they'll even go back to churning out 4E versions of all those Forgotten Realms and Ebberon books (ie. shifted by 100 years for the case of FR).