Star Trek: 12 Subplots That Went Nowhere
6. The Warp Speed Limit
Star Trek: The Next Generation's episode "Force of Nature" was a forgettable, but well-intentioned effort by the writing staff to tackle an ecological issue and end the story with lasting consequences. The seventh season episode revealed that warp drive was threatening the very fabric of space and concluded with Starfleet enacting a warp speed limit to help mitigate the environmental damage.
The warp speed limit subplot certainly had the potential to shake up franchise's status quo, but writer Brannon Braga rightly concluded that "when you limit warp drive, the rug is being pulled out from under Star Trek" and the storyline was thankfully dropped. The warp speed limit would go on to be mentioned in two more episodes following "Force of Nature" ("The Pegasus" and "Eye of the Beholder"), both times in order to give the Enterprise-D permission to exceed that limit, and then never talked about again.
While behind the scenes material indicated the variable geometry warp engines employed by the USS Voyager in Star Trek: Voyager were intended to counteract the issues raised in "Force of Nature", this was never spoken on screen and never accounted for in Star Trek: Deep Space Nine or the TNG movies.