There's some episodes of tng that change fundamentals of the universe: Parallels - everything that can happen, will happen The chase - everybody is a descendant of the same alien race Force of Nature - warp drive has the possibility to damage space. Fun episodes but I kind of ignore their implications (since the show more or less ignores it too and treats them as one-offs). Parallels has really disturbing implications like the overrun by the borg universe. What other episodes do this and do you like the changes they make?
Second Chances - Riker is merely one of two copies of the "original" Riker. It creates some unfortunate implications about the technology. Are you really the same person after going through a transporter? Couldn't this duplication mistake get replicated intentionally in the future, and for nefarious reasons? Kinda like The Prestige.
Good thread! We should have one like this in the original forum, too. In TNG, I think one could be The Pegasus. Not only does it show a game changing technology that Starfleet, or parts of it, are sitting on, but it's a major international (intersellar) incident because Picard blatently decloaks in front of the Romulan showing them the treaty violation.
Emergence - this is a strange one. The ship's computer and circuitry gains some sort of consciousness and spontaneously creates a life form .... somehow. In The Measure of a Man, they define Data as a life form, but also balk at the idea of a ship's computer refusing to undergo a refit because it is property. So what now? The computer has already shown it can create sentient life in Elementary, Dear Data (Moriarty). Are they accidentally showing that Starfleet's technology has gotten too advanced for them to deal with? A simple misspeak can create life.