Robin Hood, vampire, Paul Revere ("Escape from Boston"), Titanic, and Jeff's LGOP sequel didn't get much further than "treatments," if I remember correctly.
I think "Abyss" actually had a fair amount of code written before it was cancelled.
The adult infocomic by Elizabeth Langosy was tentatively titled "Haitian Honeymoon," and was cancelled when Infocom (really Mediagenic) and Tom Snyder Productions fell out. It was substantially complete, and I thought it was pretty good.
The German version of Zork was completed, but our play testers in Germany reported that the language was very stilted and formal and didn't fit well with the style of the game in English. We had hired a highly respected professional translator, but I think she was more familiar with business and technical translating, so the result wasn't up to our standards.
Brian Moriarty's SF RPG was just a fragment when he left Infocom for Lucasfilm.
Restaurant at the End of the Universe is a special case. At one time or another I think every Infocom author was slated to work on Restaurant. Douglas had written some bits of it, and I think Stu actually did code the opening scene (on Magrathea). Fundamentally, though, Douglas was too busy to work on it seriously, and a little bored by the whole HHGTTG thing at that time. He also felt more inspired by some authors than others, and so even less got done when the ones he wasn't inspired by were assigned to it. This was one reason why Bureaucracy was written first, and that one had a cast of thousands on the authoring side, including a "script doctor" or two.