It’s possible Moffat is eyeing Gillan for the role of Clare Abshire, which sounds like the type of high-level casting HBO likes for its shows.
For example, he’s working on an adaptation of The Time Traveler’s Wife for HBO.
It’s a possibility, but let’s also remember Steven Moffat has his own projects in the works and may be recruiting Karen Gillan to check her availability or level of interest in some things he has in the works.