@MLClark I started work decades ago on an alt-universe Civil War story -- until I found out that lots of people had already done one.
I still have the chapters, so I might pick it up one day ...
I seem to recall a DS9 scene with Bashir and O'Brien discussing temporal paradoxes. O'Brien said (paraphrasing), "I just don't think about it" or somesuch.
@MLClark I'd been tipped that Paramount was looking for a story to bring back Denise Crosby. (Turned out they went the Sela route ...) I started writing one that was a "Yesterday's Enterprise" sequel, but it got turned down.
The Tasha who left on the Ent-C turned up with Romulan refugees. No one could figure out who she was except Guinan, who had that perception beyond linear time.
(Thanks to the retcon Ribbon ...)