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The introduction serves to inform the reader of the basic premises, and then to state the author's thesis , or central idea. A thesis can also be used to point out the subject of each body paragraph. When a thesis essay is applied to this format, the first paragraph typically consists of a narrative hook , followed by a sentence that introduces the general theme, then another sentence narrowing the focus of the one previous. (If the author is using this format for a text-based thesis, then a sentence quoting the text, supporting the essay-writer's claim, would typically go here, along with the name of the text and the name of the author. Example: "In the book Night , Elie Wiesel says..."). After this, the author narrows the discussion of the topic by stating or identifying a problem. Often, an organizational sentence is used here to describe the layout of the paper. Finally, the last sentence of the first paragraph of such an essay would state the thesis the author is trying to prove.  The thesis is often linked to a "road map" for the essay, which is basically an embedded outline stating precisely what the three body paragraphs will address and giving the items in the order of the presentation. Not to be confused with an organizational sentence, a thesis merely states "The book Night follows Elie Wiesel's journey from innocence to experience," while an organizational sentence directly states the structure and order of the essay. Basically, the thesis statement should be proven throughout the essay. In each of the three body paragraphs one idea (evidence/fact/etc.) that supports thesis statement is discussed. And in the conclusion everything is analyzed and summed up. 
You present a persuasive argument for the abandonment of the 5-paragraph essay and suggest a new format for presenting an argument... However, you did not provide much information on what is expected to go in between your introduction and conclusion? You suggested this format opens the essay to compare/contrast, cause/effect, analysis, etc... but how do you suggest students structure an essay with these approaches in practice? Any piece of writing needs some structure and main ideas that are then supported with various pieces of evidence (whether you are writing a historical thesis or a persuasive essay)... If you are abandoning a "main idea followed by supporting evidence" format, what do you propose should take its place? Or perhaps my understanding of the 5-paragraph essay you are speaking of is incorrect?