I'm kind of glad they did keep it to two episodes; the effort to stop the Borg getting as far as starting to assimilate Earth is a pretty neatly constrained story. But it's fascinating to think what could have been, and how much longer arcs could have come to Star Trek years before they came to that format in later series.
The Worst of Both Worlds, a four-part arc in DC Comics' TNG comic series. In that story the Enterprise-D crew and pulled into an alternate timeline where the Borg did indeed start to assimilate Earth, and eventually help the alternate Enterprise-D finally stop the Borg, some years after their initial attack! If you're curious, IDW reprinted the whole story in their Best of the Borg omnibus a few years ago.
For many more behind-the-scenes documents and photos from TNG and TOS, check out Mission Log's Documents section. And if you're looking for more behind-the-scenes stuff from The Best of Both Worlds, check out the recently revealed photos of the unique ships scene in the wreckage from the Battle of Wolf 359.