Are there any live action TV shows or movies that ended years ago that you would have liked to see a continuation of but due to the length of time it has been, you couldn't see working today without replacing the cast or doing a reboot, which might not work with a new cast? I say live action since people age and it is more noticeable than voice actors in an animated series.
One I can think of is Stargate Universe. That got cancelled and we never got to see a conclusion to it. There has been talk of the Stargate franchise coming back at some point, and I know there are fans who would like a continuation of Stargate Universe, but given that it was cancelled in 2011 and the cast are 14 years older now, I just couldn't see it working unless they rebooted it with a different cast and I don't think that could work well either.
Another is Star Trek Enterprise to give us the 5th season we never got but without the original cast reprising their roles, I don't think it could work and as was cancelled in 2005, too much time has passed. Even the last episode of Enterprise, "These Are the Voyages..." which was set during the TNG episode "The Pegasus" you could see that Jonathan Frakes had gained weight since that episode and also Marina Sirtis playing Deanna Troi looked different to how she was in TNG.
An example that actually went that route: The Series of Unfortunate Events (Lemony Snicket) book series, which diehard fans BEGGED for a movie sequel for years, got yanked around by the director consistently expressing interest but total silence from Paramount/Nickelodeon/etc, long after the cast had aged out of the roles (the kids only age 1 year in the 13 books). Also, Jim Carrey not wanting to do sequels. They thankfully went the Netflix TV show route with a new cast, which was a success.
Las Vegas, one of many shows that got screwed over by the writers strike. I think at least one of the cast members passed away and the others are too busy for a continuation to ever happen.
Also Sonny with a Chance with Demi wanting nothing to do with Disney anymore and her being too old for the role anyways.