I would expect the retired Queen to be Mother Winter. I believe she gave or lost her walking stick to the Councils "Black Mage".
What Snark said: Per WOJ the Mothers change by Abdication; Mother Summer has done so once in Human history, Mother Winter never has.
To the OP: There's been nothing official that I know of.
I do know of two theories currently that I (personally) think have any meat on them:
1)Harry's Bargains. Per WOJ, that third Favor is still in play, but largley moot because the Winter Knight thing basically gives her the ability to ask infinite favors. The theory goes that the Last Favor might be used in a case where Harry might need to be sent after somebody that, per the terms of his Knighthood Bargain, she cannot send him after, specifically a Loved one. So if she needs to send him after Molly or McCoy or Thomas or somebody she cannot order him as her Knight, but she can still ask the Final Favor. And since the terms of those Three Favors said that afterward he'd be entirely Free of all Sidhe influence, all obligation to Lea and Mab (by the technical wording), the theory is that it might negate bargains made after that including the Knighthood.
2)
Tam Lin is a folk character that was mentioned as a previous Winter Knight, and if you read the tale through the lens of the Dresden Files, it seems to boil down to "actions taken at a Wellspring on Halloween won him free of the Fairy Queen's service and made him mortal again". It's incomplete and non-specific, but the pieces that are there sure fit the proper form.