2. William Shatner In Star Trek: Enterprise
At various points during Star Trek: Enterprise's run, there were rumours about William Shatner reprising his role as James T. Kirk. Before both the third and fourth seasons there were official statements about how ideas were being formulated and considered etc... but it seems that it was only in the fourth season that things were ever serious. You see, William Shatner wanted to be in it. He really did. So much, that he was pitching his own ideas. Shatner's big idea wasn't even to appear as Kirk he wanted to appear as Kirk's counterpart Tiberius from the Mirror Universe. It was effectively a nod to Shatner's Shatnerverse books where the Mirror Universe features fairly heavily. The plot would have been that the Tantalus device didn't actually kill the people placed in it it simply threw them back in time to another universe (the main Star Trek universe). After the Mirror-Kirk returned to his universe following the events of Mirror, Mirror, at some point Mirror-Spock would have usurped him and used the device on him. So he appears older in Enterprise, having survived on some planet for several decades before being discovered by Archer and the Enterprise NX-01. The problem was that this would cause the main timeline to know about the Mirror Universe ahead of the events of Mirror, Mirror. Although, I maintain it would have been a cool episode and perhaps given at least one version of Captain Kirk a death scene that the character was worthy of. Hell, perhaps Kirk could have been Archer's own personal version of Khan. Parts of the concept survived into In A Mirror, Darkly the USS Defiant is pulled back in time into the Mirror universe from the main universe. I even think they offered Shatner an alternative I vaguely remember something about being an ancestor of Kirk who has to pretend to be him for some mission. Wisely, Shatner turned them down.