1. A computer-implemented method comprising: under control of one or more computing devices configured with specific computer executable instructions, identifying a plurality of original objects in an item of original content; selecting an original object comprising at least one original attribute from the plurality of original objects; selecting, from a source other than the item of original content, a replacement object comprising at least one attribute; replacing, in the item of original content, at least one original attribute of the selected original object with at least one attribute of the selected replacement object to form an item of modified content; determining a coherence value for the item of modified content, wherein the coherence value is based at least in part on a number of times that two or more words included in the item of modified content appear together in a plurality of items of original content; and based at least in part on a determination that the coherence value does not satisfy a threshold value, replacing, in the item of modified content, the at least one attribute of the selected replacement object with at least one of: a different attribute of the selected replacement object or an attribute of a different replacement object.