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I think a pro. demo (not a showcase/devdemo/preview) should be taken from the nearly finished game.

A demo is made to motivate the player buying the product.
Therefor a demo should show the main idea, the graphics, the control, the sound, ...

If a demo should show ALL features of the gameplay depends on the game and on you.
A game could be quick get boring if you have no new features after the player bought it as well as you have a different gameplay with the full version.

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Yeah, but there is still the storyline, more weapons, more levels et cetera..