It's interesting because on one hand I like the idea of having a reason to visit certain areas because there's opportunities for different activities, where NPCs might provide different services, but when those services are sort of secondary to gameplay it might be better to keep them as a function that can be performed in many locations; our ship for example. I know in that game you could have people move into your buildings to provide services. I don't know if Destiny's ships will be that accommodating, and something about going to an upgrade shop in the city every time seems like it could get tedious. Like we saw in the E3 demo, you can upgrade your gear between combat encounters, like after the fireteam defeats Rixis. We don't know how large your carried inventory will be but it's certain your ship will store your complete arsenal, and it's been hinted that your Ghost will be your link to your ship. So if something like more in depth weapon crafting is a thing, maybe you could call in your ship when you get to an appropriate LZ, do some crafting, and then continue on in the area, rather than having to exit and fly back to the city. That cuts out the loading time while maintaining plausibility as you're not just carrying around a workbench and tools to take apart, maintain and reassemble all your guns. If areas like the city are social hubs it makes sense that non-combat activites should be there, to "slope the floor" from doing idle, RPGish things toward partying up to go on missions. Shopping for or winning items in minigames or light (non-combat) competition could be some of those activities, but maybe more for aesthetic things like skins for gear and weapons.

I'm not really familiar with Terraria but I think it's unlikely Bungie would expect players put time into crafting consumable items like ammunition. I don't know but it seems like you're thinking we'll have unlimited ammo, in which case having upgrades or special ammo types would make a lot more sense. Despite being a loot/loadout-based rather than a sandbox-based weapon design (you don't get to pick up and use weapons that alien enemy NPCs use) you actually don't have unlimited ammo for your weapons. There was some confusion about this based on cues in the scripted E3 gameplay demo. You do actually have to replenish ammo on the battlefield. Ammo box drops from the enemies are generic to the weapon class; one box refills your main weapon, another your special/secondary weapon, and another your heavy weapon. If you watch the E3 gameplay videos carefully you'll notice the ammo pickups, despite that they had infinite ammo turned on for the purposes of the demo. No sense having ammo drops if all the weapons are designed to have unlimited ammo by default. So you're going to be burning through lots of ammo, and it just wouldn't make sense to spend hard-earned resources on something you exhaust so quickly. Rather, ammo upgrades like exploding rounds and such seem tied to the weapon upgrades themselves (see Thunderlord), plausibility be damned. As far as attachments, in the pic above you'll notice a couple icons separate from the Thunderlord's main upgrade path that could represent attachments that came with the weapon, which may be able to be swapped out for other attachments. That could suggest at least some kind of modular swapping of parts, if not being able to build a gun from the ground up with modular parts.