I'm expecting the game to essentially bring together elements of various previous games, along with a few new features.
To be clear, what I'm saying is that items and world features will transfer over. I'm betting that the Ocarina, or a new instrument functioning in the same way, will make an appearance, complete with ability to control the day/night cycle (OoT/MM) and the direction of the wind (WW). I'm expecting a multitude of "godlike" beings scattered around the world, much as seen in Wind Waker with Valoo and Jabun, with a number of them distressed or altered in some way due to the evil forces in the game.
One thing I'm really hoping for is the reappearance of a number of races that haven't been used enough. The Kokiri have been in just one game (unless you count Fado in WW - I don't count the Koroks as the same race, just as being related), so I'd love to see them return... this game could perhaps be set in the Wind Waker timeline, but prior to the events leading to the Great Flood. The game could include the original Sages of Wind and Earth, and perhaps also reveal some of the story of how the Gorons came to be what they were in WW.
The game is supposed to break a number of Zelda conventions. One could be that Ganondorf is the big bad guy pulling the strings (with Skyward Sword substituting that with a character that is effectively proto-Ganondorf, to provide some mythology for the rebirth of the Zelda and Link souls). Indeed, it could be interesting to see both Link and Ganondorf substituted for some new characters, while keeping Link in the game as an NPC or supporting playable character. Obviously, Zelda has to be in it as a main character, since it's a mainline Zelda title, and the series is called "The Legend of Zelda" (spinoffs can break that rule, but mainline titles kind of have to have Zelda).
I'm expecting Warp points (either like OoT with dedicated songs/items/etc, or like MM with single song/item and set of markers, or something new). I'm expecting the return of a number of classic items other than the given ones (given are bow, boomerang, sword, shield, bottle, bombs, potions, fairies, and something along the lines of a hookshot). I'm expecting a fire rod, some equivalent of the Quake Medallion, the shovel, some form of hammer, and the Lens of Truth, at least.
And as for how to fill the entire world... there are so many possibilities. Free DLC, paid DLC, etc, are certainly options. So is the option of having dedicated dungeons that have items obtained at the entrance and then used throughout the dungeon, only to be given up upon departure. This could allow them to be a lot more creative in designing items and dungeons, without having to consider the impact of those items on other dungeons. And of course, NPC towns, etc. If they take a bit of inspiration from how the Pokemon games handle their NPCs, it wouldn't be too overwhelming. Heck, they could probably go through and create hundreds and hundreds of NPCs that can be put somewhat randomly into the world.
One thing I'm betting returns is the daily schedule for many NPCs, as seen in Majora's Mask. That's one feature that is always raved about, and Nintendo seem to be trying to capture that sort of thing (features people rave about, that is) with this game. It could have multiple schedules, including multi-day schedules, allowing for a more dynamic experience, where sometimes an NPC will be found within their house all day, and other times they take a trip out into Hyrule Field to visit their friend (who may or may not be there when they get there, producing a variety of possible scenarios).
Seriously, it wouldn't be hard to fill the world, if they're clever about it.
WHO IS JESUS REALLY?
