The head writer said that filming is currently under way on episodes three and four, a two parter by Toby Whithouse, but his episodes, presumably including The Magician's Apprentice, will be filmed next.
The executive producer then teased:
We’ve got some other stuff lined up from other writers – some new, some old, some middle-aged.Moffat also said that it was too early to be teasing too much but that there is,
Very, very good stuff coming.[Source: Doctor Who TV]