I really enjoyed when TCM showed DAVE BRUBECK IN HIS OWN SWEET WAY (2010) a few years ago-it was excellent.
I also think TITLE KING (2008) about Saul Bass would be great to see on TCM. They could show WHY MAN CREATES (1968) as a double feature with it.
What about the fun SEARCHING FOR DEBRA WINGER about today's women's complaint there's no good roles for women outside of sexpots? And the wonderful THE KID STAYS IN THE PICTURE about filmmaker Robert Evans?
They certainly have "classic movies" tie-ins.
Interesting "making of" documentaries that are often part of Criterion sets are also an untapped source for TCM. Although most are a total bore*, sometimes movies with special effects offer interesting behind-the-scenes view on movie making. The often shown POLTERGEIST one TCM shows as a filler comes to mind.
*I understand they are often made for insurance documentation purposes.
And as much as I love THE ARISTOCRATS, it's not suitable for TCM.