One of the goals of FO is for things to be done/made available via Players, not NPCs, so things like NPC merchants, and of course, any kind of Quest giver shouldn't be added.
Some window-dressing NPCs wouldn't be a bad thing, and of course there are the NPCs that were part of some of the Kickstarter plans that would fit in. Mayhaps a Town Cryer that the Gov/VG can set to give messages of import to the town/guild could be a thing.
The 'empty' feel of Fractured is mainly because population numbers are currently very low. That's in part because we're amping up to a wipe, and partly because it being a Testing phase (currently closed Beta) people did what they came to test, then moved on until new stuff was dropped into the game, perfectly natural. Sure, some of us die-hards stayed, and that's good for the game too, but your going to see that population ebb and flow during testing phases. Hopefully, after launch, you still see an ebb and flow, but with a greater concentration of players during peak times.
I am, in other ways, not really in favor of too many NPCs outside of mobs within the FO framework. It's one of the features that attracted me to FO