There are people living inside the TravellerPeople or aliens may operate the Traveller like a spaceship or habitat rather than the Traveller itself being a single entity. This leads into...
The Traveller's residents will appear as a faction in the future
The Traveler is related to Cocoon in Final Fantasy XIIICould be another similar construct located on a distant world (with more friendly Fal'Cie responsible for the Traveller's powers).
The Traveler is responsible for humanityís fall from graceThis could be one of three ways:
The Traveler is Durandal after the closing of the universeHe escaped to a parallel timeline, and decided he would protect humanity.
The Security Officer from Marathon will appearWhat was the name Durandal gave him, as the universe closed? Destiny
Destiny is set in the aftermath of the Human-Forerunner warThe Traveler could be a Forerunner Warrior-Servant who has taken a liking to humanity - doesn't that sphere look an awful lot like a Cryptum?
A major questline will be to track down and neutralize someone trying to enter the Traveler and deactivate it.Because that big of object, floating that low above the ground? If it shuts off, it is going to fall unless the Guardians stop it from doing so. It's practically inevitable that someone will try. Of course, that paves the way for major secrets to be revealed about the Traveler, not least of which is locating a suitable door.
The Traveler is sentient, and will be helping you throughout your travels.The E3 demo revealed an AI ally, named Ghost (voiced by Peter Dinklage) who tries to help you. ("Tries" in that during the demo he turns on the lights, which alerts all the baddies). WMGs have been built on less.
The Traveler came from the SunThis poem from Alpha Lupi, an ARG website launched by bungie, associates the sun with hope and returning strength, the same themes associated with the Traveler: The Blaze sits inside a nest of little worlds, still too distant to share its heat but plainly staring out at you. A face emerges, drawn from plasmas and radiation... There must be meanings in its roar. You listen hard and carefully, and sometimes a lucid melody seems to rise out of random noise, pulling your mind into moments where it seems possible that answers are about o be revealed. Joy builds, and the first hope in ages transforms you. It seems important, even critical, to tell every star from here to the black between the galaxies that you are strong again.
There will be an AI called JoyeuseIn The Song of Roland, the inscription on the blade of the sword Cortana read; "My name is Cortana, of the same steel and temper as Joyeuse and Durendal". Bungie's last two sci-fi game series had a Durandal and a Cortana respectively, so they're probably going to complete the set sooner or later and it may as well be in Destiny.