Which book, movie or television show have you loved that ended too soon?  Is there something that you wish had a sequel, spin off, or new episodes?  If they did it - tell us what that would look like, what would the show/book/movie be about?  Anything goes!

Defiance.

Ascension 

Hannibal. Dexter. House. Halt and Catch Fire. Bates Motel. 

They all ended beautifully but I wasn't ready to let them go. =/

Sons of Anarchy, The Sopranos, The Black Donnelys.

This isn't really possible now and wouldn't really have been possible at the time due to financial concerns, but I do wish Logan was a trilogy of films instead of a single movie.

It's pretty close to perfect as it is, but I think more time exploring this "alternate universe", more scenes between Hugh Jackman and Patrick Stewart, and stretching out the final act of the film and exploring those secondary characters more would have been excellent.

Firefly without a shadow of doubt

The Black Donnely's was gone to soon.  *nods*  

As for MASH would you have it set in a new war?  I loved the series and it's perfect in every way really.  One of the best shows on TV - EVER.  But what would you do with it now?

And yes... Logan was damn near perfect, but I'm with you in that I really could have gone for a trilogy.  :)

Sliders, the first two series had a funny twist on possible realities e.g if Russians overtook the USA or If men were trying to get equal rights with females;but from season 3 new writers and a twist for darker action sci-fi to fill a void spoilt it for me.

Someone else already said it, but Firefly, for sure! I wouldn't mind more Buffy too, but they continued that in comic book form, so it's ok.

My name is Earl and Flashforward

FireFly yes they need to keep that show going. 

Spartacus and the soprano's

Systematic Immolation of Thought

Now that TJ Miller is no longer on Silicon Valley, I would like to see an Erlich Bachman spin off

The Dresden Files.  That show was really good. :D