1. Star Trek: The Lions of the Night
Throughout the 1990's there were rumours of a Captain Sulu led spin-off series, but it seems that was all they were - rumours. However the closest Sulu came to sitting in the Captain's chair again was for a proposed series of CGI animated movies bridging the gap between The Original Series and The Next Generation. Around 2003 work began on a project called Star Trek: The Lions of the Night, it would've seen Captain Sulu and the Enterprise-B holding back an invasion by the Kzinti (a race of aliens first introduced back in The Animated Series episode The Slaver Weapon). Concept art was produced, and a story was put together by writer Jimmy Diggs, but it seems that the decline in viewers for Star Trek: Enterprise meant that Paramount pulled the plug before this one got any further.
2. Star Trek: IMAX
This proposed 40 minute movie went into production in 1997. A budget of $12million was set, and a script written by Rick Berman and Hans Tobeason. Star Trek: IMAX would've been shot in 3D and made full use of the IMAX format. The story was to star Colm Meaney as Miles O'Brien, David Warner reprising his role as Chancellor Gorkon and small cameos from characters from TOS, TNG, DS9 and Voyager. Both Paramount and IMAX loved the idea but Berman claimed that unknown "business reasons" meant it was put on a back burner, and eventually dropped completely.
3. Star Fleet Academy
Theorising that it would be much cheaper, and logistically simpler to create a prequel adventure for Kirk and Spock, instead of having to negotiate both salary and availability with William Shatner and Leonard Nimoy, producers Harve Bennett and Ralph Winters began work on a Star Fleet Academy movie to celebrate the franchise's 25th Anniversary. The script was approved, featuring an introduction scene with Dr McCoy (DeForest Kelley) addressing a class of graduating students at the Academy, the film would then flashback to the original crew's early days in training. Something JJ Abrams would show many years later.
The film never happened because Paramount decided they wanted something more traditional for the 25th Anniversary, and Star Trek VI: The Undiscovered Country was produced instead. It was also a very unpopular idea amongst many of the original crew (obviously so, they'd be out of a job), and even Gene Roddenberry himself, who in a later interview said:
"I didn't like it. Who was going to cast the new Kirk and Spock? No one has ever cast a Trek character besides me that's worked. Braggadocio or whatever, that is the history of Trek. It wasn't good. Some of it was like Police Academy. You could hardly do this without the magic of a group of characters tailored for Star Trek, which this was not."Tell that to JJ Abrams!
After Star Trek was cancelled in 1969, Paramount were keen to produce a new-spin off series for broadcast the following season starring Leonard Nimoy. The idea was to have Mr. Spock take a sabbatical from Star Fleet and be 'dropped' off on his home planet of Vulcan. From there the show would solely concentrate on his adventures with the other Vulcans, presumably being very logical. Paramount asked Gene Roddenberry to create a treatment and produce the show, but he turned it down and the project faded away.
5. Gary Seven - Assignment: Earth
The earliest proposed Star Trek spin-off dates from 1968 and has its origins in the season 2 episode Assignment: Earth. Ratings for Star Trek itself were poor and it very nearly didn't get a third season order, so Gene Roddenberry was already planning his future post-Trek. He pulled out a story he'd written back in 1966 about Gary Seven, a mysterious character who had been sent by advanced beings to help Earth in the 1960s. At this time it didn't feature any of the Star Trek characters at all, but with a little bit of re-writing Roddenberry had a Star Trek script that was intended to act as a backdoor pilot for a new series starring the character of Gary Seven. A last minute third season order of Star Trek ended this idea idea, but the characters featured did go on to appear in a comic book series from IDW Publishing.
Nathan is getting too old too quickly and is rapidly approaching his pipe and slippers phase.