“Star Trek Enterprise” had a “Miror, Miror” episode, I wonder why TNG didn’t.
But they did. There was an episode with the Enterprise-C (can’t remember the title). In that episode, because the Enterprise C falls through a hole to the time of the Enterprise-D, it isn’t around to save the Klingons at some conference. As a result, the D’s time is changed. The Federation is no longer a “UN in Space”, but rather something closer to the Klingon Empire. And Tasha Yar didn’t die. She actually ends up going back with the Enterprise-C to the past, since she was supposed to be dead anyway, and becomes the mother to a half-romulan version of herself that comes back later to trouble the Enterprise-D.
/Nerd