The moment ST: Enterprise aired and someone realized "Hey we can easily do a Borg ep based on First Contact!"? AND airing "Yesterday's Enterprise" and the realization it could be used to bring back Denise Crosby into the main timeline. (Either as an older Yar or a relative)
IIRC Denise Crosby pitched the idea of Tasha's daughter herself. IDK how long after YE that was, though. As for the ENT/Borg thing, it was probably when they decided to spin the series off of the events of First Contact. It's a no-brainer.
Well, are you interested in how stories are broken or how fast they're inspired? What prompted your question?
On the DVD commentary for Regneration, Mike Sussman says he was saving the basic story idea for a point when they ran out of ideas. When that finally came, he was hesitant to share it, for fear it would be liked and he really didn't know how he would feel about that.
Curious about any anecdotes, such as "Did the writers of 'Yesterday's Enterprise' immediatly realize that this could set up a return of Tasha Yar down the line or (Since 5 years passed between FC and ENT) "did they go in producing ENT fully aware that there was a ready-made Borg ep there to be used?"
Honestly, I really hope that while planning a prequel series to TOS the thought "hey, we can bring the Borg in no problem" was not instantly thought up.
I remember member Samuel Cogley predicted there would be a Borg episode before the series even premiered and was roundly shouted down. Wonder whatever happened to him? Haven't seen him post here in ages.
Well, I seem to remember everyone making jokes about when the Borg would show up on Enterprise, due to "Bermaga's" perceived lack of writing skill. As for Cogley, I know he was around last year, at least. I remember him making a joke in the Trek XI forum about "Cumberbatch Kids."
After watching the pilot, I remember one of my sister wondering how long until the inevitable "Space Nazis" would show up. We all laughed at the very idea.