You know, given all of the stuff that's flown around, I'm surprised
nobody's come up with one VERY obvious possibility.

When Tom Baker announced he was leaving the series, he made an interesting
hint, more out of a joke than anything else, that the Doctor's next
regeneration could possibly be female.  This created a mild uproar at the
time, and left to some speculation as to what sort of actress could play
the Time Lord (Lady).  

I'll let all of you out there put the rest together.  

-- Nick