Possible identity of River Song
Okay...been listening to the Big Finish, Eight Doctor series. In listening to Zagreus & Scherzo (with Scherzo being the most mind bending Who story yet!. Scherzo is the bastard child of the move "Altered States" & "Eraser Head").
You really need to hear/read both these stories together.
Anyway. The Doctor's affection towards Charlotte "Charlie" Pollard seems to go way beyond that of Tennent & Rose.
She's his best friend & they openly claim to love other .
My theory is that the secret name she called the Doctor was Zagreus, as with all the Gallifreyans dead, no-one else would know that name. I also propose that River is actually Charlie.