This is all a bit speculative, so let's lay out the facts first. Doctor Who producer Marcus Wilson posted the following Tweet yesterday: "Diederick Santer gave me lots of good advice. The best - always leave the party while you're still having fun." Diederick Santer is a television producer who has a long and storied career with the BBC, including a four-year stint as Executive Producer of EastEnders. He left shortly after the high note of the 25th Anniversary episode to go start a production company of his own by the name of "Lovely Day". Various Twitter followers assumed this comment means that Wilson's leaving the show; Wilson hasn't posted a followup confirming or denying either way. So he might have decided to go pursue other opportunities, or be thinking it over; presumably Steven Moffat would want to know now, before production on the Anniversary Special and Series Eight begins in earnest.

Marcus Wilson is one of only three people to have consistently received Producer credit on the show since the Series Six launch of "The Impossible Astronaut", the other two being Denise Paul and Steven Moffat himself. (Brian Minchin did some script editing for Series Five, but has been working as producer on various other shows since then, including The Sarah Jane Adventures, Torchwood, and Russell T. Davies - Phil Ford creation Wizards vs Aliens). If he does leave, Moffat is essentially going to be breaking in an entirely new production team for the show's next series. And as of yet it's still not clear whether Faith Penhale is staying on as Executive Producer after her stint for the 50th Anniversary Special...so it would appear to all be very much up in the air. Time will tell.