Why is th\at there is no sequel to titanic????

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WE have seen many movies which are made part by part and the success goes to the starting movie ...and the sequels make money on the reputation of the first one..... But still titanic being such a huge hit there hasnt been any reports of being it made in a second like it happens with most of the hollywood flicks....yet i heard some rumours and gossip of JACK being coming alive.....but none of them concrete.... Do you think that a sequel to the TITANIC can be made.???? If yes then what should be the main theme of the story....