What does Mr. Tuck dream in Tuck Everlasting?

Mr. Tuck dreams that his whole family never drank from the spring, died, and went to Heaven. 

Some people may fantasize about being immortal.  Being immortal, Mr. Tuck dreams of dying.  He lives in an endless trap, where he can never age and never live a normal life.  He wants to get off the wheel, as he calls it.  This is the fantasy he dreams of. 

Tuck twitched and the smile vanished. He opened...

Mr. Tuck dreams that his whole family never drank from the spring, died, and went to Heaven. 

Some people may fantasize about being immortal.  Being immortal, Mr. Tuck dreams of dying.  He lives in an endless trap, where he can never age and never live a normal life.  He wants to get off the wheel, as he calls it.  This is the fantasy he dreams of. 

Tuck twitched and the smile vanished. He opened his eyes. "Why'd you have to wake me up?" he sighed. "I was having that dream again, the good one where we're all in heaven and never heard of Treegap." (Ch. 2) 

Mae Tuck tells him it is no use to dream this dream, because nothing will change.  He tells her that he dreams it every day, and that he can’t help it.  The dream is a manifestation of his desire to end his immortality, but it is also a coping mechanism.  In a way, he dies every night but then wakes up, if only symbolically. 

Tuck explains the stress of immortality to Winnie in the rowboat, after she accidentally sees his son Jesse Tuck drink from the spring and they take her home with them to tell her what the spring does. 

That's what us Tucks are, Winnie. Stuck so's we can't move on. We ain't part of the wheel no more. Dropped off, Winnie. Left behind. And everywhere around us, things is moving and growing and changing. (Ch. 12)

The Tucks have been immortal for a long time, and have made peace with their existence.  However, it can still be hard.  While Mae enjoys the times when they are all together, when Jesse and Miles return, Angus often gets frustrated with the constant day to day mundane life.  He wishes that the Tucks could be like everyone else and die.