Hiya!
The Keep on the Borderlands is my all-time favourite module of all time. And not just because it was the first one I ever played. Out of all the modules I own (which is probably 75%'ish of all 1st edition AD&D and D&D modules), this one is the most bang for your buck in terms of raw, untapped, open-ended fantasy'gasm for a campaign start.
I have run multiple multi-year campaigns with this puppy (as in 2+ years each, and nobody ever leaving the area of the "wilderness map" pull out of the module). Each one with most of the same exact group of players. And each campaign was totally different from the others. How many adventure "modules" can pull that off? Not many. I think Isle of Dread and maybe The Secret of Bone Hill could stand next to KotBL and not feel ashamed.
If you want to see what and how much the KotBL could be 'expanded', you have to try and track down a copy of Kenzer & Co's The Little Keep on the Borderlands, for Hackmaster (4th Edition...not the 'newer' version...the old one based on 1e/2e). It clocks in at almost 150 pages, chalked FULL of adventure. Everything in it is expanded, without being all tied and linked together so tightly that removing one unravels the whole thing. The organized humanoids in this thing are organized. And the Kobolds? Don't. Just...don't. Go try and find the ogre or minotaur....those kobold bastards will kill you dead before you even see them or their cave! Honestly, I can't say enough good things about this HM4 adventure. Find it, buy it, play it (preferably in Hackmaster 4, but hell, any 'D&D' version would work...just remember to remove 20hp from everything...it's a HM thing...).
Oh, I thought of a more recent adventure that I'm actually in the process of 'converting'; The Night Below. The area around Harenshire is one of those "mini-campaign areas" that can be used for an entire campaign. So many cool areas and things to explore! A slight re-imagining of this boxed set for 5e would rock on toast! Mmmmm.... Hopefully my pitiful scratch notes do it justice when we start it in a few sessions (I don't suspect the current 'bad guy' campaign is going anywhere; we just don't really like playing bad guys...but the players had an itch and we are currently scratching it; won't last much longer, I figure).
^_^
Paul L. Ming