I think that the "Gallifrey is missing" will be an ongoing story thread much like "The Last of the Time Lords" was for much of RTD's and SM's tenures. It will be there, in the background, and will provide the backstory and, from time to time, motivation for the Doctor.
With "The Last of the Time Lords", over time we learned that it's because there was a war against the Daleks and both species were destroyed (excepting the Doctor himself, of course); oh, and then we learned a Dalek had survived; and then a few Daleks; and then the Master; and then.... well... you get the picture. We were drip fed bits through both tenures, until SM decided to "blow the doors off" with "Day of the Doctor".
I think we've had just one reference to the "Quest for Gallifrey" in series 8 and it will be drip fed slowly like that for several years.
If SM is wise, he'll leave the final stages of the search (and perhaps the rediscovery of Gallifrey) to the next show runner.
The problem with bringing back Gallifrey full time, and the Time Lords fully restored, is that the Time Lords are too powerful. It'd be too easy for them to create a problem, or fix one, "deux ex machina" style. This wouldn't make for good drama.