Text generation using word/sentence/phrase-level ad-libs is so simple its a common homework assignment for first-year programmers. You really don't need much.

Text generation using word/sentence/phrase-level ad-libs is so simple its a common homework assignment for first-year programmers. You really don't need much. There's even tools like Tracery that let you construct grammars and vocabularies as a form of textual generative art.