Last year, Star Trek: Discovery became the sixth series to continue the spacefaring mission of Gene Roddenberry's original '60s Trek, following in the footsteps of The Animated Series (1973-74),The Next Generation (1987-94), Deep Space Nine (1993-99), Voyager (1995-2001) and Enterprise (2001-05). Not to mention the movies.

But these were far from the only Trek series developed, pitched or planned in the near-50 years since The Original Series departed our screens.

Here are some of the most epic-sounding shows we'll never get to see – reboots, sequels and spin-offs that never materialised. (No pun intended.)

1. Star Trek: Phase II

Paramount Pictures

Trek creator Roddenberry had planned a sequel to TOS –Phase II was officially announced in 1977, with an order for a two-hour pilot, followed by 13 more episodes.

Most of the original cast were set to return, with the exception of Leonard Nimoy, who turned down the opportunity to return as Spock. In his place, a new Vulcan character named Xon was created, with actor David Gautreaux cast in the role.

There were also questions over William Shatner's involvement, with fears that he was too expensive to retain permanently, and another new character, Willard Decker, was created as a potential Kirk replacement.

The project was abandoned when the CEO of Paramount Pictures decided that the planned pilot would work better as a feature film, which eventually became 1979's Star Trek: The Motion Picture.

Two of the other scripts written for Phase II would also later be redeveloped and used for The Next Generation.

2. Star Trek: The Original Series reboot

Paramount

In 2004, J Michael Straczynski, the man behind Babylon 5, and Bryce Zabel, co-creator of cult '90s sci-fi series Dark Skies, planned a Trek reboot featuring the original crew.

Their pitch was broadly similar to the Star Trek film reboot that reached cinemas five years later under JJ Abrams. But at the time, Paramount dismissed the idea and, according to Straczynski, were not "even willing to talk about Star Trek."

3. Star Trek: Federation

Franziska Krug/Getty Images

Following the cancellation of Enterprise in 2005, Bryan Singer and his frequent collaborator, writer Christopher McQuarrie, developed an idea for a new Trek series along with producer Robert Meyer Burnett and screenwriter Geoffrey Thorne.

Federation would have explored an era where the Federation was in decline, in an attempt to bring back the "rough and tumble" feel of the original series. "I pictured a Federation that had hit its plateau and stayed there for three hundred years," Thorne explained. "Utopia has occurred and everything has stagnated."

Thorne wrote a pitch document detailing the show's eight lead characters and outlines of the first four episodes, with the intention that McQuarrie would write the pilot and Singer would direct.

But these plans were scuppered when Paramount announcing development of Abrams' Trek movie reboot in 2006 and the proposal was never given to the studio.

4. Star Trek: Final Frontier

Brice Parker

Another project quashed by Abrams' movie, Final Frontier was a planned animated series, in development for the web and announced in late 2006.

Set in the year 2528, after the events of the final Next Generation film, the show would have chronicled a war between the Federation and the Romulans, with the crew of a new Enterprise – led by one Captain Alexander Chase – as the main cast.

CBS expressed interest over the proposed series, but like Federation, it came to nothing once Abrams started work on his Chris Pine / Zachary Quinto-starring reboot.

5. Assignment: Earth

Paramount Television/ CBS Television

The final episode of the original Trek's second season introduced Gary Seven (Robert Lansing), an alien from the 24th century sent back in time to protect Earth's history from outside interference.

This episode (what's known in the business as a 'backdoor pilot') was intended to set-up a spin-off featuring Seven called Assignment: Earth.

Gene Roddenberry had hoped to devise more standalone adventures for Gary and his team, human assistant Roberta Lincoln (Teri Garr) and... erm... a cat named Isis – but the planned series was never greenlit.

6. Hopeship

CBS / Paramount

One more spin-off that wasn't to be, Hopeship was devised by Roddenberry and writer Darlene Hartman – again, during the production of the original Trek's second season.

The show would have played like a Trek medical drama, charting the voyages of a Federation hospital vessel. Dr M'Benga, a recurring Trek character played by Booker Bradshaw in two episodes of the original series, would apparently have featured as a regular.

Like Assignment: Earth, though, the idea ultimately went nowhere.

7. Harry Mudd

CBS / Paramount

Intergalactic con man and smuggler Harcourt Fenton "Harry" Mudd was a popular recurring character on Star Trek, with NBC expressing an interest in giving actor Roger C Carmel his own spin-off in the '60s.

According to Carmel, Gene Roddenberry turned down the offer, since he was too busy working on the flagship series: "You couldn't blame Gene, he didn't want to let somebody take it off in a direction he didn't approve of. Since he didn't have time to handle it all, the Mudd series project died."

The character did eventually get a second life, of sorts, with Rainn Wilson replacing the late Carmel in recent episodes of Star Trek: Discovery.

8. Star Trek: Excelsior

Paramount Pictures

Ever since George Takei's Sulu was given his own command, the USS Excelsior, in 1991 film Star Trek VI: The Undiscovered Country, fans have been campaigning for the character to get his own spin-off.

Takei was willing to bring the idea to Paramount, but there was apparently little interest. "There was a huge following for it," he later vented. "But for whatever reason, Paramount didn't pick up the idea. So despite that massive and heroic effort that was launched by all of the people, the idea didn't go through. I was absolutely baffled."

9. The Worf Chronicles

CBS via Getty Images

Michael Dorn, the actor who played Worf in both The Next Generation and Deep Space Nine, dreamt up a spin-off for his fan-favourite Klingon character. "I think he's a character that hasn't been fully developed and hasn't been fully realised," Dorn told Star Trek News in 2012.

"Once I started thinking about it, it became obvious to me that I wanted to at least put it out there, which I have, and the response has been pretty amazing. We've been contacted by different individuals – I can't say who and all that – about wanting to come on board."

Three years later and Dorn insisted that the announcement of Star Trek: Discovery was "probably the nail in the coffin" for the project. Poor Worf... denied yet again.

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