The idea of a spirit who was haunted in life (as Master Gracy is suggested to have committed suicide) opening up his home to all these other wayward spirits who cannot pass on for some reason and giving them a place to rest and enjoy themselves. He could just as easily have kept to himself.
The film versions' ending, where thanks to the Evers family, all the spirits save Ramsley are freed.
"Angels in Heaven, together at last. The tale is well ended for those who have passed. Love endures all, no reason, no rhyme. It lasts forever and forever all the time."
Ezra and Emma waving goodbye.
The fact that they left with Madame Leota and the Singing Heads.
Since he was such a fan-favorite character, the Imagineers brought back the Hatbox Ghost, with advanced effects that allow him to work! Many fans are overjoyed about his permanent re-addition, and some even claim that he completes the Mansion.
The Ghost Post manages to make one that is also rather creepy. Apparently, all of Constance's husbands still love her, even after she murdured them. And she seems to at least enjoy their company.