This is totally and completely a rumor, but it would be so cool if it turned out to be true. The folks at Den of Geek report:
Sci-fi goddess Gillian Anderson is being lined up to star in the fifth series of the popular television show Doctor Who, according to the Daily Express.
The actress, best known for playing Dana Scully in The X-Files, is reportedly being lined up to star as The Rani in Matt Smith’s first series as the Time Lord. The Rani is a renegade Time Lady who first appeared in Who during the Eighties’ story Mark of the Rani and returned in Time and the Rani. She was originally played by Kate O’ Mara.
Considering The Doctor is supposed to be the last of the Time Lords it sure seems like there’s quite a few of them popping up, but I’ll let it slide for the awesomeness of the idea of Anderson playing The Rani.