Something about that little cutscene with the probable Queen of the Reef tells me the Reef will be a lot more like the City than not, gameplay-wise.

Bungie Quarians.

And it looks gorgeous.

It seems the most likely way to expand the game to me. The solar system is vast and there are lots of places that we could go :)

As that bit thing says in Tron, yesyesyesyesyesyesyesyesyesyes. Keep it to places we actually know something about so the fiction is way better. Like hey Bungie, we've got probes on the way to Jupiter and Pluto, hang off on those places until some of that sweet, sweet info is in eh?

Go nuts on Saturn though. Cassini so pimp.

While I still think there's more in the initial game than those 4 locations, I sort of like your idea of expanding outward later on. Whether that's in DLC or later games... well, whatever.

It would make sense in that case that, in addition to Earth, we're starting with the Moon, Venus, and Mars. They are our closest neighbors.

Does that leave Mercury as a possible undiscussed option for the first game? Since it's closer to the inside of the solar system than those other locations, it doesn't make sense to me to have it be a later conquest. Or, maybe Mercury's just too inhospitable even for the Traveler's terraforming, and there's nothing there to find. :)

In the recent New Beginnings trailer the narrator says that "Mercury became a garden world..." Plus we've seen a player ship making its loading screen flyby of Mercury in previous ViDocs or whatever.

Well, both The Traveler and The Darkness came from beyond our system So, somewhere in the heavens... someone is waiting for us...

The story is an investment system.
Possibly we do not get to explore the outer planets in the first game? They have a much talked about '10 year story arc' that they have to pay off? Keeping part of the solar system 'off limits' for the time being might be in the best interest of the story and the successive content or titles that may come?

The solar system is being overtaken by The Darkness. Maybe we have combat Darkness and use the Traveller's Light, over time, to reveal planets and their moons after which they become "accessible" to us? A bigger, badder, evil awaits us out there. Or possibly the greatest of Evils hangs quietly, low above The Last City...waiting...

There's also the possibility that in later games we finally leave our Solar System and carry the fight elsewhere. After all, that has been hinted at, and really, humanity's empire is hinted as having extended beyond just our one little System, and now it is our goal to reclaim all that we have lost - that would include other Systems.

So, here's one possible line-up:

Destiny 1 - We are limited to a select number of planets, probably from Mercury to Saturn.
Destiny 2 - We now can access the entire Solar System, all the way out to Pluto.
Destiny 3 - We now have access to both our Solar System and a couple of other, smaller systems.

Yeah, I agree with your line of thought, and I'm more thinking something along the lines of your first possible lineup.
However, I know there's lots of talk out there about this first Destiny title including Jupiter, Saturn (or at least their moons), and the other planets in some way in some way, but I'm not pinning my hopes on that. I'm being conservative for now. For the first title I'm sticking with what we know is in the game at this stage; Earth, The Moon, Venus, Mars, The Reef, and possibly Mercury. That's already a huge amount of content; anything more in this first title is a massive plus for me.

P.S. Poor old Pluto isn't technically a planet anymore, however I guess it could still be in considering our Moon is.

The solar system is being overtaken by The Darkness. Maybe we have combat Darkness and use the Traveller's Light, over time, to reveal planets and their moons after which they become "accessible" to us? A bigger, badder, evil awaits us out there. Or possibly the greatest of Evils hangs quietly, low above The Last City...waiting...

Big fan of the Distrust-the-Traveller camp. And I would expect our range in the Sol system to be limited until we bring the light of the Traveller to other points in-system. Makes plenty of sense. Whether those locations are simply opened up by completing missions, or whether they're opened up in successive rounds of DLC, I wouldn't have any issue.

I would be bugged, however, if we never got to see all the areas within our own system within D1.

Here's another possible line-up:

Destiny 1 - We are limited to our Solar System, but we have access to everything there (at least, everything within reason - I doubt we'll be able to walk on the surface of Jupiter).
Destiny 2 - We escape our System and make it to a couple of others, where the bulk of the difficult combat is - there'll probably be some alien fringe worlds here.
Destiny 3 - We start attacking the enemy Core Worlds, and probably even their own Homeworld[s].

This is the idea I like best.

A lot of it depends on the nature of the Traveller, though. Is it merely some kind of autonomous uplift drone? I guess it's all storyline content that we've yet to see, but I want to know whether the assistance is overt, or merely just a convenient byproduct of the Traveller's presence.

Wouldn't that be something if we meet the Traveller's creators somewhere down the line and they're astounded that we even exist, that we struggled and fought in the shadow of their deluxe Roomba and prevailed?

~m

Destiny 1 - We are limited to a select number of planets, probably from Mercury to Saturn.
Destiny 2 - We now can access the entire Solar System, all the way out to Pluto.
Destiny 3 - We now have access to both our Solar System and a couple of other, smaller systems.

I like this option a lot more!

There are tons of cool things to see in our own Solar System! It's seems pretty common for (large-scale exploration style) science fiction video games to take place in other solar systems. I think there's a lot of room in Destiny to uniquely blend science fiction and reality, which is a strong part of what makes the world of Destiny both "mysterious" and "intriguing". It's that familiarity, yet weirdness, that makes exploring the worlds of Destiny so interesting and Destiny would seem like a very different game to me if that feeling were completely lost in future installments.

Indeed. Heck, did we even know there'd be a new Halo in Halo 2?