G1-2-3: Against the Giants/ Odds 3:1
A giant-centric story is heavily suspected by the community given plot points laid out in previous adventures. Given Paizo has recently run-down their own giants-themed AP it'll be interesting to see if Wizards follows suit sooner or later.
S3: Expedition to the Barrier Peaks/ Odds 5:1
Crashed spaceship is another popular theory given Chris Perkins' recent teaser involving a picture of a Froghemoth and other aberrant monsters. Would Wizards really mashup scifi and fantasy in Faerun though?
The Ruins of Undermountain/ Odds 6:1
A straight up dungeon crawl. This combined with others like Greyhawk Ruins or Maure Castle is a natural option for any long term game. Given the locations of Wizard's last few stories this one cannot be discounted.
The Rod of Seven Parts/ Odds 7:1
This is another idea I've seen mentioned in forums. The epic adventure boxed set has just the right components for a contemporary D&D mega event. It has demons (too soon?) and it has a multipart artifact quest.
Gates of Firestorm Peak/ Odds 7:1
This classic module is by one of Wizards' best authors Bruce Cordell and involves the Far Realms which is always a huge meta-threat in D&D. Firestorm Peak would make a good alternative to borrowing Barrier Peaks. For bizarre monsters and crazy evil NPCs you can't do much better than this.
WGA4: Vecna Lives!/ Odds 8:1
Uber-liches are popular villains in D&D and one like Vecna who wants to supplant existing gods is definitely a Forgotten Realms type plot (Time of Troubles, ahem). This is also one of the last strongest Greyhawk modules out there that hasn't been redone since 2E.
I6: Ravenloft/ Odds 10:1
Uber-vampires are also a popular monster-villain in fantasy and TV (not to mention hugely popular zombies which you can lump in here). Normally this type of storyline would be the province of the Ravenloft setting, but maybe Wizards can spin a new yarn to rival Strahd Von Zarovich?
Scaly evil and jungle ruins is an untapped resource for D&D writers in the last couple editions. Given WotC's tendency to set their adventures in temperate or cold climates though, this type of story is still far off in my opinion.
Human traffickers is an evil organization that can keep adventurers busy across a wide area. This classic series has been revised recently so it may be fresh on Wizard's mind. It can also translate to other settings easily.
That's all I can think of off the top of my head. Feel free to add your own speculated odds to this list!