A thread of movies not yet in licensed Blu/DVD editions, none of which are controlled by major studios, but in most cases, have been held back by missing or intransigent rightsholders or other parties. Sometimes it's someone you've never heard of who's blocking an obscure fave:
DAY OF THE WOLVES seemed prime to return, with a devoted fan creating a making-of docco and even a @DayOfTheWolves account for progress reports, but the project seems to be abandoned, likely either due to Ferde Grofe's hope of a remake sale and/or composer Sean Bonniwell's death.
DAY OF THE TRIFFIDS has been undergoing a painstaking restoration by superfan Mike Hyatt for nearly 20 years; I saw an in-progress version in 2009, looked great. However, he understandably wants to be recompensed for his expenses by any licensor, and nobody has met his price yet.
BUSTER AND BILLIE, released by Columbia but financed by Ted Mann (of Mann Theatres) had been a TV and VHS staple for years. Reportedly however, a while back when Sony's rights expired, a rep for the Mann family collected all their elements, and the movie has not resurfaced since.
NIGHT OF THE JUGGLER was released by Columbia, but financed by another theatre chain, General Cinema, and did not travel to AMC when they bought GCC out. At last check, it + FOOLIN' AROUND is now owned by a bank that has no interest in making a deal to get them out of the vault.
OPEN SEASON is yet another former Columbia release that has reverted to a third party, producer José Antonio Sainz de Vicuña. Urban legend is that he rejected a $30,000 licensing/buyout offer from the late Sage Stallone because he was too busy coaching his son's football team.
THE COOL WORLD is likely the only fiction film controlled by Frederick Wiseman's Zipporah Films. If you have a spare $400, you can buy a VHS from their website for "educational purposes only." Music fees have surely been a significant impediment to making this publicly ownable.
Rudy Durand was improbably successful at suing WB, Viacom, and Melvin Simon Productions to get the rights back to TILT. Unfortunately, aside from a couple cryptic YouTube videos, he has not seemed to make any effort to get his labor of love back into wide availability again.
If you haven't followed @MovieRamblings terrific @full_julia account (you should!), he's done wonders to track down the convoluted rights history behind THE HAUNTING OF JULIA, but has been stonewalled for months by a UK lab, so he is taking a well-deserved break from the quest.