Might you be looking at How I Met Your Mother's mother?
Doubtful, but actress Rachel Bilson, has been cast in the 100th episode of HIMYM - one in which fans have been promised by the show's executive producers "lots of mother action".
Furthermore, we've been informed that in this episode "It'll be the closest Ted's ever come to [discovering who she is]. He's getting a step closer every episode this season, and episode 100 [we] kind of go bananas with it."
What role do you think Bilson will be playing? We're thinking not the mother, but perhaps some relation to her?