There's a place on Io we can't get to but can see from a distance that looks nearly identical to the Covenant Holy City, but without the Forerunner Deadnought overhead. It's likely a massive Vex construct and is comprised of what appear to be large concrete blocks arranged in concentric circles. At first glance it looks like a city. I believe the Traveler may have been above it when it was terraforming Io. There are a number of things in Destiny that are reminiscent of items in the Halo series which should come as no surprise. For example, the Vex Mythoclast is eerily similar to some Halo weapons in overall design and shape. I'm still waiting for my Titan to get the Master Chief armor set. And I'd like a Ghost that sounds like Cortana. >;)

That place on IO is the first city on IO constructed. Made under the traveller when he visited. Ikora explains this in the first cutscene when you seek her out in the story.

I never made the high charity connection but can see it. Maybe a little nostalgia influencing my liking of Titan :)