I believe Jackie Tyler is the Rani!
Why is she the Rani?Edit
Let's think about it, she doesn't like the Doctor, and he doesn't seem exceptionally fond of her! In fact, in "Journey's End," he tells her not to touch anything in the TARDIS. Why? Simple! It would jog her memories!
At some point before the Time War, she used a Chameleon Arch to disguise herself as a human. She decided to not be particularly pretty, so that the Doctor, if she encountered him, wouldn't recognise her. Sadly, she never opened the fob watch she used, and after marrying Pete Tyler, he sold the watch at a pawn shop.
After the Ninth Doctor met her, he realized who she really was, but didn't drop any hints in case it would set her off, nor to do anything to disappoint his new friend Rose Tyler. However, the Tenth Doctor made sure to seal the Rani away in an alternate universe where Jackie Tyler had been a real person, doomed to live the rest of her life as a wife and a mother, never realizing that she was really a Time Lady.