leaves the TARDIS then her replace will be played by the talented Jenna-Louise Coleman. 
Now nothing much has been said about who the character is - apart from Steven Moffet saying a few months back that The Doctor will met a "new friend", and it'll be a mystery about how they meet.
This was made me think, and I've come up with 3 possible old characters that Jenna-Louise's character may be the new version of.
1 - Susan Foreman (long shot) - The Doctor's granddaughter - she is a Time Lord so therefore it is entirely possible that she may have regeneration since their last meeting, when I believe The Doctor was Peter Davison (for the 25th anniversary).
2 - Jenny (long shot) - The Doctor's daughter - created by a DNA machine - she had 2 hearts, and was very much part Time Lord. She died (never regenerated) but was brought back to life - it is possible that during one of her advertures she died and actually wanted to regenerate this time.
3 - Romana (most possible) - A Timelord who did regenerate during one of the fourth Doctor's adventures, she left to help a planet. It is possible that she regenerated at least once since then.
I'm rarely right about these things - so I'll be very surprised if it turns out it is one of these characters.
The Fool.
