I've suggested NPCs in the past (though really, who cares what I think) but upon reflection I think the game is purer and more impactful without NPCs.
Right now, it's a game of life. Life doesn't hand you quests and reward you with loot for completing specified objectives. Life is neither benevolent nor cruel, but apathetic. Want meaning? Discover it yourself. Want something to do? Figure out what you want. That's life.
NPCs would introduce an artificiality into an otherwise relatively pure experience. They would make the game more fun, but would qualitatively change the experience.
The best solution would be to find a way to attract and keep more players.