9. Captain Robert April - Star Trek: Discovery
This one is a deep cut, one that many audience members would be completely forgiven for not noticing at all. In Star Trek: Discovery, Saru temporarily takes command of the ship in the first season. Seeking inspiration, he asks the computer to list the most decorated captains in Starfleet. The first name on the list may fly right over the heads of some viewers, while others might have been sitting up in their chairs, pointing at the screen.
Bar an appearance in the Animated Series, Captain, later Commodore and Fleet Ambassador, Robert April hasn't been seen in action. He was the original commander of the USS Enterprise, taking her out on her maiden voyage. He was also an initially planned character to appear in the first Star Trek pilot, The Cage.
Through the various rewrites that The Cage went through, April was transformed into Christopher Pike, who in turn was later transformed into James T. Kirk for the second pilot.
For the longest time, it looked as though April was lost to history. Yet thanks to the Animated Series and various expanded media examples, the idea of him never truly went away.
His inclusion in Discovery could suggest a later appearance in Strange New Worlds, which will of course be following Captain Pike and the Enterprise. Here's hoping that one day soon, Star Trek's first captain can finally appear in the flesh.