I like the idea that humans were the ones to disappear, usually those who leave behind technology in other worlds are alien races.

Perhaps a developing species was native to the planet, and humans came to colonize it, for some reason a disaster occurred, and they had to leave. All that was left behind was the hints of their technology and wastelands of what used to be their homes. The other species took refuge and developed in it. Not knowing humans were still waiting for their time to return, perhaps having a large station in orbit, regarded by the native species as some sort of deity or just another natural satellite. Or perhaps they tried or managed to return to earth or wherever human in this time live. 

We can make a mixture of technology levels, and a mixture of architecture this way too, the natives taking reference to build from what was left behind and what was shown in books and other such things humans left. We can have pockets of much more or much less developed styles here an there due to the humans visit. 

As for magic I was thinking of putting it in, the old idea of technology and magic working together is a favorite of mine. 

Just spitballing, I like what i'm seeing, it's very intriguing.